Well, first can anybody tell me the resolution and the size of the head texture to make it the best quality in-game?
Also I don't know how to explain the thing that I can't run shaders on XP, but can run them on 7
P.S. My graphics card update says that i doesn't support "current OS"

The head texture that will provide the best resoultion in previews is 512x512. However they automatically get resized to 128x128, when you upload and you are required to post 128x128 previews when selling.

I'm not sure, why you can run shaders on 7, but not XP.

Well, that's about the 7:
http://s53.radikal.ru/i139/0909/00/c7c5014ee1d7.png
P.S. You mean shaders can't be runned on XP?