Now computer won't shutdown

Hello, I just installed the latest Avast free software on my computer (XP sp3, Realtek HDA sound card). Now the computer won’t shutdown at all. This computer doesn’t have a power button, so I have to switch it off on the powerstrip. I am worried the hard drive hasn’t parked properly. I googled shutdown problems with Avast and found quite a few listings, but none that seem to apply to my system setup. This is the first web security software I have installed. There was no other security software installed. Can someone point me in the right direction? Thanks.

Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?

What other security based software do you have installed, anti-spyware, firewall, etc. ?

Are there any errors in the windows event viewer (roughly the time you shut down) ?