I've bought every level of CS package from CS2 up for my graphic designer, and there's no such thing (at least that I've never seen) as Photoshop for XP. CS4 runs perfectly fine on Windows 7. http://store.adobe.com/store/en_us/p...ystemreqs.html
Windows 7 isn't mentioned because they want you to buy CS5. Click the link about Windows 7 compatibility, and it will take you to the order page for CS5. CS2, CS3, CS4, and CS5 all run just fine on Windows 7.
If you don't want to take my word for it, run the Windows 7 Upgrade Advisor.