One other answer to the same error message (this just fixed my problem) is that the System drive was low on disk space. Meaning about 700kb free. Deleting a lot of unused stuff on this really old server and then restarting IIS and the website (probably only IIS was necessary) cause the problem to disappear for me.
I'm sure the other answers are more useful for most people, but for a quick fix, just make sure that the System drive has some free space.