I had this problem, too. And i've solved it. I think it's happen because some programs are not compatible with XP.
In my case, after installing "Easy file & folder protector", my computer can't be restarted or shut down (it freezes at the log out screen with the message: "Preparing to Shutdown", too). And when I uninstall it, XP runs normally.
So you should try to uninstall every programs that recently be installed or not necessary.