If there are not strange sound from the harddisk, normally the disk is fine.
You forgot to tell us how big is the disk and how the partition C: D: distributed?
Which year of the HD and how long is the year of use, Windows version?
Before you anything else, backup all C: and D: files to CD-R or other HD by famous ghost.
When this start to happen? how is the tempeture?
OK. what you could try after the backup is done!
1. use fdisk check the disk patition record can be displayed.
2. Record the extend partition D:'s partition table HEX info.
3. Try use new fdisk to get one partition only
4. format the disk. if still have problem, then you need to low level format the disk from bios. But it's not recommend to do, it could make your HD dead forever.
5. install at least win98...