BUMPFF,
Rickity, 25F, and TCS all have great suggestions. If after trying their suggestions, you still think it is the hard drive, I recommend getting a copy of Norton Ghost, or my personal favorite -
Acronis True Image.
The beauty of True Image is that it lets you backup straight to USB 2.0 and Firewire drives. All from Windows and without rebooting!
You could get a new hard drive and run either Ghost or True Image and clone your old hard drive. Then you can replace the old drive with the new drive and continue where you left off with your computer.
Take Care,
Richard