If you don't need cutting edge GPU-based features such as Extended Edition 3D or accelerated Liquify, blurs, etc., Phototshop CS6 actually runs on XP. It's just limited in its use of some GPU-accelerated features. Almost everything does still function.
See this page for more info: http://helpx.adobe.com/photoshop/kb/photoshop-cs6-gpu-faq.html
You can download the trial of Photoshop CS6 and see for yourself how it runs on your particular system.