Access Violation

I've installed PHP 5.2.6 ISAPI on Windows XP with IIS 5.1. It works - sort of. I get frequent 500 errors (IE) or "Access Violation" errors (firefox). Often, I cannot open phpinfo.php even once after starting the browser. Once I get the error, I can (very rarely) fix the problem by running synciwam. Sometimes shutting down the browser and restarting it works. More often, I can shut it down and restart several times to no avail. Sometimes I give up and reboot (and it still might not work). Sometimes (but very rarely) things will work fine for hours. Basically, I see no pattern. I've seen references to bugs in php that might cause this, but not for 5.2.6. Since success is so intermittent, I can't really be sure if running synciwam is responsible for anything. Does anyone have any idea how many poor souls are trying to run this combination (5.2.6 + ISAPI + IIS 5.1)? (I got no responses to an earlier plea; maybe no one is running this combination.) Are there any clues in the hex address of the error? Is there a way to determine whether this is a PHP problem, or an MS problem - or my setup problem? I'm tending to think it is PHP, since I don't see how the usual source of errors (file security) would be exercised differently when opening the same site (phpinfo). This is driving me nuts, and it's a very short trip. Does anyone have a suggestion (other than chuck XP - I'm kinda stuck with that on my desk)? Thanks in advance. herb