Is your disk NTFS-formatted? If so, then this might especially apply to you. I don't think this is a hardware issue, if your system worked previously. The problem is, that once your (NTFS) disk is corrupted, it (the file system) necessarily cannot be repaired. The disk however is probably physically OK.
Let my story begin, hopefully my painful experience might be of assistance to you.
I recently had corruption problems on my Maxtor Diamondmax+ 9 80GB (FAT32) and primarily my 250Gb WD2500JB (NTFS), after I upgraded my system from a Athlon XP2000+, VIA-based to an Intel P4-Prescott i865pe system. I had installed Windows, and after installing applications, things started to fall apart. With almost every boot I found corrupted, cross-linked etc. files. I lost nearly 25GB of data. Backing up 320GB is no easy cake.
After I transferred the WD disk back to my old Athlon system, everything worked fine again, so I knew it had to be the system. The same seems to apply to your system: the disk itself is fine, so it is something else.
Anyway, I tried everything from clean Windows XP installs to removing hardware card by card to search for conflicts. Finally I formatted the 250GB NTFS monster and my Maxtor and exchanged my Abit i865pe motherboard. Everyhthing seemed to go well when I installed windows and some applications, but then upon one of the reboots, things went corrupt again.
After a clean install and another disk format session, and after working on the computer for about 40 hours in one week, I came up with the final problem: I had installed multiple applications without rebooting after each. Many of my software suggested me to reboot after I had installed a program, but I didn't and instead installed another program. After I had installed for example 3 programs, I rebooted, but the damage was done. Apparently windows/NTFS does not like that multiple software are installed without rebooting.
At one time, the hard disk was not even recognized in BIOS, or recognized as invalid size. Fdisk didn't find the disk at all, it was only visible in windows. Also there were errors in indexes etc. Tried also Partition magic to resize the disk, Norton disk doctor etc., but nothing helped. Only formatting the disk and actually I first had to change the cluster size, because even a standard format didn't help- it was so badly messed up.
So my suggestion, if possible:
Format the disk and do a clean install. Install software one by one and REBOOT after each install. Maybe you installed some kind of update or service pack and software simultaneously without rebooting? This could have messed up your system and my experience is, that after NTFS got screwed up, nothing else than a format could repair it.