Markjan,
If you have Norton, try this:
This behavior can occur if you have the Speed Disk utility from Norton System Works 2000 installed on your system. This utility, if running in the background, blocks the use of the Disk Defragmenter tool.
This error can also occur if the file allocation table (FAT) table is damaged.
RESOLUTION
To resolve this behavior, if you have System Works, you must quit the Speed Disk utility and restart the Disk Defragmenter tool.
To repair the file allocation table, follow these steps:
Restart your computer in DOS using the Windows Me Emergency Boot Disk (EBD).
At the command prompt type scandisk, and then press ENTER.
The following message appears:
The file allocation table FATs on the disk drive do not match. Scandisk has determined that copy 1 is the best copy. If you choose fix it, scandisk will replace copy 2 of FAT with copy 1.
Press Y to anwser Yes to the message.
Scandisk will replace your damaged FAT with the undamaged copy 1 of the FAT.
After Scandisk finishes repairing your hard disk, remove the Start up disk, and then restart your computer.