You ain't the only one experiencing this http://www.informationweek.com/news/windows/operatingsystems/showArticle.jhtml?articleID=207600950

There are problems with AMD CPU's.

>I installed Windows XP SP3 last week on 4 pc's and yesterday one of them started doing constant rebooting, pausing to ask if you want to start in safe mode, etc.
>
>I was in Outlook Express checking mail. Then got out and tried to get on the internet. That's when the rebooting started. I suspect that it may have something to do with Outlook Express.
>I read about this problem in May of this year, but I thought that they would have fixed the problem in 3 months. (grin)
>Does anyone have a solution?
>I could restore the O/S to before I installed SP3, but that 's just "shooting in the dark".
>All help appreciated.
>Bill Wright