I installed every PHP-Version above 4.2.1 I could download, moved the corresponding DLLs into the correct directories and tried calling phpinfo-page 
andmy rather big PHP-CMS. Everthing works fine and fast with Versions up to 4.2.3 (unfortunatelly phpmyadmin wouldnt run with it because of that mbstring-bug that was already fixed in newer versions). With PHP 4.3+ (stable and dev versions) I still can call the phpinfo-Page but all "larger" PHP-files will either crash the Apache (1.3.27) occasionally or sometimes even deliver an empty HTML-page (although the same package works fine on Linux on the next machine). I only encountered this problem in Windows XP.