IIS/PHP w/ NTFS issue

I have IIS 5 with PHP 4.2.3 on a Windows 2000 server and I use NTFS to control access to the directories. A user with read/execute (or even Full Control in testing) can get to the directory fine, and .html files fine. But .php files do not come up - access denied basically. However, if a user is a member of the local Administrators group, they can get in to the php files great and do everything needed with php/mysql and all that. Also, if I add a user to Full Control to the C:\PHP folder, I instead get an HTTP 500 Internal Server error. Any ideas? This has been bothering me quite a bit for a while. Thanks in advance, Chris