Windows XP wont shut down from software

Hi there

I have a problem that when I go to start menu shutdown and
select shutdown the computer emediatly performs a re-start.
The only way I can turn it of is wait till the re-start
begins and power the machine off at the switch on the PSU.

This is freaking me a little, the last thing I did on the
computer was install the two last security updates for
windows XP pro from the windows update site. I will
uninstall these and see if this has done it.

I'm worried because recently my password at work became
compromised in some way and a spamming attack was started
using my work mail account, I've changed all my passwords
but can't help but think this is a bit of a coincidence. I
use a VPN to work from home.

Any advice would be very welcome, I have a third party
firewall at home, am I being parinoid here.

cheers

scr