im not a n00b but..

[church_pk]

i've been having this coninuous problem with stepmania. I load stepmania, all my songs load and everything correctly, but when i play a song, the arrows scroll is really choppy, and after about 10-15 seconds into a song, the arrows freeze, and my computer fully shuts down and reboots. I have tried re-installing stepmania, and it has not helped, i have defragged my comp, nortan virus check, and everything is fine, it's getting really frustrating now because i haven't played in days. If anyoine can help..maybe a suggestion of what the problem could be..would be really appreciated...thx