Often when I do a shutdown/restart, my system acts like it's going to restart but then doesn't. I have to shut it off by the power button and restart that way. Why is this?
Are you connected tp a Network? The reason i ask this is because one of my Windows XP computers does this if i don't disconnect it from the network before i try and reboot. It does all it's shutdown stuff, then just as the screen would usually go black - it freezes. And it's not an overheating problem, i've got more fans in there than Old Trafford on a Saturday! Maybe reinstalling Windows is the only cure. Pain in the posterior though!
there are various causes for this - mapped netwrok drives being a common one. Search the Microsoft Knowledge Base (support.microsoft.com) with the words 'shutdown hangs', work through the causes and solutions and i'm sure it will be fixable.