I agree with AB. It appears that your drive may be staggering to an untimely death. Backup anything you'd like to keep, and do it immediately.
(See this Wikipedia article on the Windows NTFS file system, metafiles section)
After making sure you have a good backup
- Open My Computer
- Right-click the D: drive
- Click Properties on the menu
- Click the Tools tab
- Click the Check Now button
- Select the first checkbox (Automatically fix file system errors)
- Click "Start"
It may notify you that it can't perform the check until Windows is rebooted. That's OK, and is normal for drives that are in use. Reboot and let the disk check run.
If the disk check doesn't come out perfectly clean, repeat the same procedure until it does. If it keeps finding errors, or finds more each time, then the drive's a goner.