Usually that sort of error is caused by a disk problem. You can try booting from e.g. a Vista installation disk, going into the repair console, and running
chkdsk /r on your server's C: drive.
Failing that, server reinstallation is your other option.
This FAQ gives an overview of the process which, if performed properly, is not destructive of your data.
I'm not on the WHS team, I just post a lot. :)