Taskbar/Start Menu Freezes

1. Taskbar/Start menu freezes. Navigating through the start menu options eventually crashes/freezes it.

2. When taskbar frozen the Desktop icons are still active and I can open those. I can also open MyDocuments folder

3. Although I can open MyDocuments folder and navigate in there, once I try to open MyComputer folder it freezes.

4. Can't shutdown computer because Start menu frozen; Ctrl-Alt-Del doesn't work - only puts the task icon in the tray but can't open it. Alt-F4 doesn't close it either. have to hold the on/off switch to shut down.

I've tried:
1. sfc /scannow (no results)
2. changed hard drive (no change)
3. antivirus, spybot, adaware all good.


I'm going to update the PSU unless someone has a better idea.

Here's computer specs:

http://support.dell.com/support/edocs/systems/dim4500S/specs.htm#1101572



Thanks for your help!!