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.