Shutdown problem !!!?

I have both windows XP professional and Vista ultimate. whenever i try to shut down my computer from XP, it restarts instead of shutting down, I didnt have such problems before, but nowadays i have it all the time i shut down from XP. but it shuts down well if i shut down from Vista.

Can anybody tell me whats wrong ? or what can i do ??

i tried through regedit
HKEY_LOCAL_MACHINE-Software-Windows-..... but i didnt find windows inside the software......

Try this;
Start-run, type regedit and click ok.
hit the + sign on HKEY_LOCAL_MACHINE-Software-Windows-Wind... NT-current version- left click Winlogon, go to the right side of the screen and see PowerdownAfterPowerdown.
If the number at the far right of the line that is on is anything other than 0, click the word in the string and change the value to 0.
Otherwise, your computer is probably getting old and the motherboard isn't too long for this world.

I dunno what's the prob, but I know a solution:

1. restart computer and shut down via Vista
2. pull the plug

try running a system restore, and if you cant go back far enough then feformat one of your partitions/drives while storing all your files (by that i mean insert the operating system disc and either try repair it or install it *which will delete your files*)

hope this helps
PS you could also try scan for viruses

how can u shut down vista???? i dont think u can r u should u click the shut down button then the restart button or it a virus u can clean the virus or reboot ur computer but back up ur info

hey see that if by mistake you have choose the option of restart on the place of shut down chek it and click on the option of shut down if already its right then format your computer.

I don't know how helpful it will be, but there's a page with tips on dealing with XP shutdown problems at http://www.aumha.org/win5/a/shtdwnxp.php