IIS PHP W3SVC error "Server Application Error" XP Pro patched to date.

[Beekeeper62]

Hello and happy new year for 2007.

I've been using PHP and MySql served by IIS on an XP machine for a year or so without problems.  Something went wrong recently, perhaps some corruption on C or on D where I have my sites stored. 
I can browse a php test page (ie just the html bit) before adding the .php for php5isapi.dll but afterwards I get the above error and see in the events manager that the s3svc cannot start.  I've un and re-installed IIS to no avail.  I expect that I have to re-install some service, but don't know which.  It takes me a good day or two's work to reinstall the whole of windows and all my apps so I'd rather not, though I have started.

Your help would be greatly appreciated.

Gregory.