Cannot Shut Down

I cannot shut down for some reason. I have tried the only things that I know, none of them work. I am running windows xp-home. I have a network going. Using Microsoft MN-100 Bridge. Motorola Cable Modem. It seems that after being on for five or six hours I cannot shut down. What am I missing here? Please Advise.
Thanks.
Wayne- Reno,NV.