This accelerator does not perform well at all under Apache 2.2.0. Just running one of my site used nearly all the memory (30M max).
The segment feature doesn't really work on Windows, so you're limited to 30M.
Update02/2006: Under Apache 2.0.55 and 2.2.2 it seems to perform normally.

eAccelerator seems faster then APC. But if you are running multiple instances of Apache on the same server you will appreciate a second caching tool for PHP. I use both APC and eAccelerator with production sites. APC seems to be getting better just in the last 3 months (since 01/2006).

Note: This accelerator might get better, in the near future, since some of the PHP developers appear to also be involved with APC.

Windows binaries download:
http://pecl4win.php.net/ext.php/php_apc.dll
Sitebuddy APC Windows Binaries for PHP 5.
Sitebuddy APC Windows Binaries for PHP 4.

Download Source: http://pecl.php.net/package/APC
Doc: http://us2.php.net/apc

Install is very easy:

1) Download the APC dll (php_apc.dll) see previous link "Windows binaries download".

2) Copy this DLL to your php extensions folder.

3) Enable the this extension in your php.ini by "un-commenting" or adding the line:
extension=php_apc.dll

4) Add APC configuration directive to your php.ini, like:
apc.enabled = 1
apc.shm_segments = 1
apc.shm_size = 64
apc.optimization = 0
apc.num_files_hint = 1000
apc.ttl = 0
apc.gc_ttl = 3600
apc.cache_by_default = On
apc.slam_defense = 0
apc.file_update_protection = 2
apc.enable_cli = 0
apc.stat=0
; If you experience intermittent blank pages don't set apc.stat=0

Following is the official description of the configuration directives.

5) Restart your web server.

------------------Official description of the APC configuration directives --------------------------------------------------
apc.enabled This can be set to 0 to disable APC. This is primarily useful when APC is statically compiled into PHP, since there is no other way to disable it (when compiled as a DSO, the zend_extension line can just be commented-out).
(Default: 1)

apc.shm_segments The number of shared memory segments to allocate for the compiler cache. If APC is running out of shared memory but you have already set apc.shm_size as high as your system allows, you can try raising this value.
(Default: 1)

apc.shm_size The size of each shared memory segment in MB. By default, some systems (including most BSD variants) have very low limits on the size of a shared memory segment.
(Default: 30)

apc.optimization The optimization level. Zero disables the optimizer, and higher values use more aggressive optimizations. Expect very modest speed improvements. This is experimental.
(Default: 0)

apc.num_files_hint A "hint" about the number of distinct source files that will be included or requested on your web server. Set to zero or omit if you're not sure; this setting is mainly useful for sites that have many thousands of source files.
(Default: 1000)

apc.user_entries_hint Just like num_files_hint, a "hint" about the number of distinct user cache variables to store.  Set to zero or omit if you're not sure;
(Default: 100)

apc.ttl The number of seconds a cache entry is allowed to idle in a slot in case this cache entry slot is  needed by another entry. Leaving this at zero means that your cache could potentially fill up with stale entries while newer entries won't be cached.
(Default: 0)

apc.user_ttl The number of seconds a user cache entry is allowed to idle in a slot in case this cache entry slot is needed by another entry. Leaving this at zero means that your cache could potentially fill up with stale entries while newer entries won't be cached.
(Default: 0)


apc.gc_ttl The number of seconds that a cache entry may remain on the garbage-collection list. This value provides a failsafe in the event that a server process dies while executing a cached source file; if that source file is modified, the memory allocated for the old version will not be reclaimed until this TTL reached. Set to zero to disable this feature.
(Default: 3600)

apc.cache_by_default On by default, but can be set to off and used in conjunction with positive apc.filters so that files are only cached if matched by a positive filter.
(Default: On)

apc.filters A comma-separated list of POSIX extended regular expressions. If any pattern matches the source filename, the file will not be cached. Note that the filename used for matching is the one passed to include/require, not the absolute path. If the first character of the expression is a + then the expression will be additive in the sense that any files matched by the expression will be cached, and if the first character is a - then anything matched will not be cached. The - case is the default, so it can be left off.
(Default: "")

apc.mmap_file_mask If compiled with MMAP support by using --enable-mmap this is the mktemp-style file_mask to pass to the mmap module for determing whether your mmap'ed memory region is going to be file-backed or shared memory backed. For straight file-backed mmap, set it to something like /tmp/apc.XXXXXX (exactly 6 X's). To use POSIX-style shm_open/mmap put a ".shm" somewhere in your mask. eg. "/apc.shm.XXXXXX" You can also set it to "/dev/zero" to use your  kernel's /dev/zero interface to anonymous mmap'ed  memory. Leaving it undefined will force an  anonymous mmap.
(Default: "")

apc.slam_defense On very busy servers whenever you start the server or modify files you can create a race of many processes all trying to cache the same file at the same time. This option sets the percentage of processes that will skip trying to cache an uncached file. Or think of it as the probability of a single process to skip caching. For example, setting this to 75 would mean that there is a 75% chance that the process will not cache an uncached file. So the higher the setting the greater the defense against cache slams. Setting this to 0 disables this feature.
(Default: 0)

apc.file_update_protection When you modify a file on a live web server you really should do so in an atomic manner. That is, write to a temporary file and rename (mv) the file into its permanent position when it is ready. Many text editors, cp, tar and other such programs don't do this. This means that there is a chance that a file is accessed (and cached) while it is still being written to. This file_update_protection setting puts a delay on caching brand new files. The default is 2 seconds which means that if the modification timestamp (mtime) on a file shows that it is less than 2 seconds old when it is accessed, it will not be cached.  The unfortunate person who accessed this half-written file will still see weirdness, but at least it won't persist. If you are certain you always atomically update your files by using something like rsync which does this correctly, you can turn this protection off by setting it to 0. If you have a system that is flooded with io causing some update procedure to take longer than 2 seconds, you may want to increase this a bit.
(Default: 2)

apc.enable_cli Mostly for testing and debugging. Setting this enables APC for the CLI version of PHP. Normally you wouldn't want to create, populate and tear down the APC cache on every CLI request, but for various test scenarios it is handy to be able to enable APC for the CLI version of APC easily.
(Default: 0)

apc.max_file_size Prevents large files from being cached.
(Default: 1M)

apc.stat Whether to stat the main script file and the fullpath includes. If you turn this off you will need to restart your server in order to update scripts.
(Default: 1)