More details here, when do you see this? What's the exact message?
If you can't start Windows at all, boot of a XP CD and run a chkdsk. This error can be caused by a bad drive, a virus, file system issues. More times than not, when I run across files that can't be found on a drive, if you do a drive test you will find errors on it, even if Windows runs after a chkdsk repair.