Symptoms: When you try to access your template admin pages within Drupal (/admin/themes) you get a blank page.

Under "administer » logs" you might see the error:
call_user_func(Copy of phptemplate_templates): First argument is expected to be a valid callback in E:\DomZeus\wwwroot\modules\system.module on line 572.
 

This happened around Drupal 4.7.3.
Basically if you have a file like "Copy of phptemplate.engine" in your "wwwroot\themes\engines\phptemplate you will get that error.
Backup that extra file and then delete it. You should be back in business...