If it is an AMD processor, there is an easy fix:

http://msinfluentials.com/blogs/jesper/archive/2008/05/08/does-your-amd-based-computer-boot-after-installing-xp-sp3.aspx

Here is an excerpt:

"The problem is that HP, and possibly other OEMs, deploy the same image to Intel-based desktops that they do to AMD-based desktops. It also appears that this is unique to their desktop image, and any HP AMD-based laptops are unaffected by the problem. Because the image for both Intel and AMD is the same all have the intelppm.sys driver installed and running. That driver provides power management on Intel-based computers. On an AMD-based computer, amdk8.sys provides the same functionality. Microsoft points out in a Knowledge Base article that installing both drivers on the same computer is an unsupported configuration, putting the blame on the OEM that deploys the image."


>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