Two clicks from http://www.aumha.org/articles.htm is the XP Shutdown article.
Here's what you'll have to note. There is no "do this" to cure your issue. As to the ROXIO software, you may be unaware that WMP (Windows Media Player) can install a piece of that for you without you knowing. To pass up a fix because you haven't installed Roxio may have left a fix untried.
Ooops, that's the magic word. You have to "try" each fix.
I have tried the microsoft fix that is posted on their techie site about removing the registry keys. One problem: i dont have the said keys as i have never had the roxio software that supposedly causes xp to restart rather than shutdown. if anyone has any idea how to fix this i would appreciate it. I have taken it to a techie already and he was unable to help me saying it was a microsoft software issue. so i went home and checked the site with no luck. Thanks. (and i'll keep checking the site)