I had overclocking issues in windows 7 where I had not had them before. Really, I'm getting RAM errors ever since I upgraded my computer to the new Phenom II and at first thought it was an overclocking error on the CPU but at regular speeds it happened. What did I conclude? I was getting errors due to the fact that my CPU can now push my GFX to it's full. How does this relate? Well if you weren't using x64 before, you might be having RAM issues with your GFX as people tend to know 64bit os's use more computer ram but also more video ram. Now that may seem silly, but the video card still has to store data, and with the bits registering as twice the size.... It's not always twice the RAM used, but it's more taxing on the ram as apposed to 32 bit and that could be a factor in what's going on. =) Just an idea thrown around there, as I didn't add it together until after I played around with everything. No really, it just sort of hit me as to why my card can OC higher before then after. As that might also be why some people get higher overclocks on 32bit as apposed to 64bit I think.
Oh and is your XP 32bit?