XP... :3 I still don't see how people cand stand using it... Vista run perfectly fine for me.
I'd say update your comp. specs. You could use a better processor, more RAM and a better graphics card.
What I'm thinking of (at the moment... I'm at work so I can't do all the things I could do at home ) is that your graphics card isn't able to render certain textures. Do you have another graphics card that you could use to test?
The 5700 is 7(?) series back, which is quite some time ago.
Doing minor tweaking can help.