Hi,
Try to use the CGI mode of php4. I've been hearing rumours about php4isapi.dll not being developed completely and bugs can occur.
I've encountered a few access violations which actually dropped my php build forum dead at the spot, coming back on after that and then going dead again.
The error I received (from PHP) in my browser was that PHP caused a access violation.
So I've configured the server to use the CGI mode of PHP (I'm using php4ts.exe) and it seems fixed now.