Did an XP install and ran into some problems.

AMD APP SDK stopped having openCL support for XP for any version past 2.5
Catalyst display driver dropped openCL support for xp for any version past 12.1

Check the display driver compatibility of 12.1 for your video card. if it doesn't work, you'll need win7
Check here http://support.amd.com/us/kbarticles/Pages/AMDCatalystSoftwareSuiteVersion121.aspx

Note: Catalyst 13.2 BETA drivers include AMD APP SDK, which will keep you from having to download it seperatly
http://support.amd.com/us/kbarticles/Pages/AMDCatalyst132BetaDriver.aspx

GPU-z is great for checking openCL compatibility. Run it, and there is a little checkbox if it's working.
http://www.techpowerup.com/downloads/SysInfo/GPU-Z/