Windows 2008 and IIS 7.0 have new features and optimizations that allow PHP to run more efficiently and robustly. The most significant improvement Microsoft made was to update the CGI support in IIS 7.0 to conform to the Fast CGI standard. Fast CGI saves a process create/destroy operation per request by pooling and reusing multiple worker processes. This translates into significant performance improvements for PHP applications on Windows.
By default, the Fast CGI support in IIS 7.0 is configured to be conservative when allocating resources in order to best support the scenario where hundreds of web sites are running on a single physical server.
From Tuning Windows Server 2008 for PHP.
Personally, we've just setup PHP (using the Web Platform Installer - I'd recommend you do the same) on one of our corporate servers, and outside of an increase in memory usage due to a couple WordPress sites, we've experienced no other noticeable issues.
I've personally installed PHP on two of my servers, and again have experienced no issues, although I don't really use PHP on those machines.
So, realistically, the only impact should be from the PHP applications running on the machine.
Can I ask why you want to install PHP on this machine? Is it to move existing apps over to the server, for experimentation, or ... ?