PHP 4.x is sometimes unstable in certain configurations on windows.
I've had two users of my product report that php will sometimes have random behavior and crash. Others have mentioned that it runs fine as CGI but not as an ISAPI module.
That said I installed and ran it perfectly fine on my win2k/php4.0.3/apache setup.
You can take a look at php.net's bug tracker and see how many crashing issues there are with windows. Hopefully it's been improving with new releases.