A portion of our site is done in PHP and a portion of our site is done in ASP.Net. We just set up a new web server with Windows Server 2008 R2 which has IIS 7.5 installed.>
ASP and PHP can be used on windows boxes. As long as they're completely separate and aren't dependent on each other. For example, using query strings (i.e file.php?var=1&var2=bla)
things get messy when you need to transfer those variables over to the ASP file or vice versa.
So as long as the 2 systems are totally independent of each other, then it should work fine.
You may also find some incompatibility with cookies and sessions. Those too can be passed but not easily.