This is very odd. Do you use the "read metadata on load" option in Winamp ? The only times I've seen winamp crash was because of some plugin... so even if it came with Winamp and even if you're not using it at all, it may be doing stuff on the background -- so I HIGHLY suggest you go through the winamp plugins and remove as much as as you can. I would also try switching the output plugin between directsound and waveout to see if it makes any difference. Short of the above I would have assumed it could be a driver issue, but since you switched from XP to Win7 (and obviously are using different drivers) I would doubt this is the issue.
Maybe you want to load some GPS application or some video encoding software and let it run while you play music in winamp and see if you have winamp issues like that -- if the Higher CPU and/or Memory usage cause the problem, this could be a hardware issue.