Ok, so this is something new, then.
Backup whatever valid files are left on the drive, then stick it in your PC and format it to NTFS/FAT32/whatever it is your operating system uses.
(Assuming you're running Windows 2000 or later, you can do this by right clicking "My Computer" and selecting "Manage" from the context menu. The "Drive Management" section of the window that appears allows you to format the drive.)
Once that's done, right click the drive icon in Windows Explorer, select Properties on the context menu, and give the error checking tool a go (it's under the Tools tab). Make sure you tell it to do a complete sector scan.
Once it's done, look in the Event Viewer (right click My Computer => Manage) for a log with the "WinLogon" source listed (I think it'll be in the Application section, to memory). This should show you the results of the drive scan. If it reports a single bad sector, write the drive off as defective, and replace it.