IIS/ISAPI require suddenly stops working

Hello, I'm using PHP 4.2.3 on IIS as an ISAPI module (for testing only). So today I'm using my scripts just fine and then all of a sudden, require stops working. It returns Fatal error: Failed opening required 'C:/Inetpub/public_html/smarty/Smarty.class.php' (include_path='.;c:\php4\pear') in C:\Inetpub\public_html\index.php on line 27 Everytime I try to require or include something it gives me the same error. And it only gives it to me with that file. The same exact file includes perfectly on Apache/Linux and other files include just fine. I tried deleting the contents of the file and leaving only a <?php ?> and it fails. I tried renaming the file and it fails. Like I said, all other files work, but this one file doesn't, no matter what it's contents or what it's name or where it's located in the directory tree or how many times I recreate it in different places. I am stumped. ~Paul