Windows XP restarts when I try to shut down. This, according to Microsoft, was a problem if you have Roxio Easy CD creator 5 installed. I do not have this. When I shut down my computer it always restarts again but shuts down at the second attempt if I do not go into any programs. No error messages appear but it is annoying.
It sounds like (but might not be) a program that you're using is opening a device which could be used to restart you PC. USB, Network, Firewire.. something like that. Before you shut down do a "ctl/alt/del" and see what's still running. It may be taking the initial attempt to shutdown to close down completely. (Then again it might not be, but it's only a suggestion at this stage)
I had a similar problem but had Roxio Easy CD installed. Eventually found that the trouble was caused be an Adaptec combined USB and firewire card. This card caused the problem even with the drivers uninstalled. Card went in the bin - expensive experience.
i had a similar problem cant remember exactly but somewhere i think in advanced power managment theres a tick box that says restart computer if incorrectly or fault on shutdown something like that.if you untick the box it will stop rebooting it worked for me anyway.darren