xp incredibly slow/grey bars of doom

Up until 3 days ago my computer was running fine and has now become exceedingly slow at startup (about 2 hours!). Even after Windows has started the computer just seems to slug through (opening 'my computer' took about a minute). I looked at similar threads but none seemed to be exact to my problem. I updated my copy of Norton's and ran a virus scan which found nothing. I ran Scandisk which took about 2 days to run through and fixed several issues. The same startup issues remained. I checked msconfig but I only have 4 programs that are loading with startup. I checked my hardware in 'my computer' but everything appears fine. I also bought a copy of (and ran) System Mechanic. It found 1267 registry issues which I repaired. I restarted but the same problems exist.

The computer starts up fine, the Windows splash screen appears, and then I get the black screen with the grey progress bar that slowly ticks along. Any ideas? I have 2GB of DDR2 Ram and a 500 GB HD with a 1 TB back up drive. I have windows XP. I had not made any changes to the system in the past month. This setup has worked flawlessly for the past 2 years and has just recently been giving me grief.

Thanks for any help