In all honesty, that sounds a lot less like a driver problem and a lot more like malfunctioning hardware. Actually seeing the corruption in question might help narrow it down a bit more, though.
I regularly run very intensive OpenGL apps in windows on my ATI card with no issues. While it's certainly possible your benchmark app hits some particular driver path that isn't as stable as what my apps hit, the "runs fine for a bit, gets corrupted looking, crashes" is almost the textbook prime symptom of an over-heating GPU.
It could be the case, CCC on WinXP is overclocking the R520 and making it unstable. I never checked that CCC for ages now.