You told me you were using fastcgi, and I am I am curious if using php as an isapi extension might work better.
I have been playing with options as we look at possibly moving to an iis server, and the isapi extension is reported to be faster and more robust than the regular cgi. There are others who know a lot more about this than I do, and perhaps someone will weigh in.
This is a quotation for a XOOPS site I found as I was trying to figure this out for myself. I have no experience here, although I may by the end of the school year.
"With IIS for example (as IIS is my area of expertise) you get much better performance under high loads if you configure you PHP application mapping in ISAPI mode rather than straight CGI due to the fact when using CGI the web app is loaded into and out of memory for every single request, where as with ISAPI the web app is only loaded into memory once where it can be used again and again for all requests. Then on top of that again you can greatly increase PHP performance by using an caching/accelerator such as Zend, mmcache or eaccelerator. You can also get further performance gains through performance tuning your MySQL environment, but I haven't explored that to any great depth at this stage."