Stuart Herbert, prompted by this post from Derick Rethans uses this new post to his blog to point out something that didn't seem to be mentioned and has always been a pet peeve of his when running PHP on IIS - controlling the FastCGI processes so they don't take over the machine.
Running PHP via CGI and FastCGI means that IIS has to do the Windows equivalent of fork()ing off PHP processes to do the actual PHP bit. If your box has too many PHP processes running, the box will start to swap. Once a webserver starts swapping, you've no chance in hell of keeping up with all the incoming requests, and your websites on that particular webserver become unavailable in a matter of moments.
The problem seems to have been corrected in the most recent IIS release, though and correct directions can be found here. Older versions of the web server are out of luck, unfortunately.