Remove any flopppy drives, CDs, USB drives and try to reboot.
Try running the chkdsk utility from command prompt during boot up. On Dell computers I believe you have to press F8 twice. Then you should be able to get to the command prompt and enter chkdsk /f and this should check the drives.
If this doesn’t work, you may want to try the Ultimate Boot Disk CD (free).
I came across this suggestion for a fix but I haven’t tried this myself, so try it at your own risk.
1) Boot from the Vista DVD.
2) Click ‘Repair my Computer’
3) At the prompt to select your OS installation, click the button that says ‘Load Drivers’. This should bring up a explorer-style drive browser.
4) Then just right-click on the drive you tried to compress, and deselect ‘Compress this Drive’, which should still be ticked on. Hit apply, and apply to subfolders/files.