'ere's a funny thing...

[John Marsh]

Microsoft article may be of use:
Windows XP restarts unexpectedly or restarts when you shut down the computer

Plus other comments at PC won't sleep or shut down, only restarts | Ask Metafilter

SD: I've had to use the power button method on occasion. Sometimes this makes XP check files & folders on the next boot-up. I've also known it to mess up the desktop. (But when nothing else works, what can you do?)