I'm sorry to say this but the fact that it's saying that you will need to run chkdsk means that the harddrive is dying. This is common (as all harddrives will fail sooner or later) and though running chkdsk can repair it temporarily, the symptoms progressed so quickly on your system that I believe that if we tried doing anything to the drive, we may be causing even more harm to it.
The best thing to do would be to purchase a new harddrive, load XP on it (yes, it does run much better on less capable hardware than Vista does), and add the harddrive with Vista on it as a secondary harddrive on the computer.
This way, you can try recovering what you need off the failing drive and have a good drive to copy it all to.
If however purchasing a new harddrive is not an option, and you can obtain a Vista DVD so we can run the chkdsk scan, we can attempt to recover data off of and essentially quarantine the bad sectors on the drive which may bring your system back to life, though I wont be able to guarantee it would last long after that.
Right now you want to avoid turning the PC on as any time the harddrive disks spend spinning may be making the problem worse.