/etc/mtab should be on a local drive, the error message suggests it's on a network (NFS) share.
Could you do
and post the results?
What is interesting, is that /etc/fstab can be opened (normally mounts are on folder-level, so fstab & mtab would be part of the same mounted
filesystem, except if mount --bind were used). If there is a remote share involved, a reboot of the NFS server (or restart of nfsd) & a reboot of your machine should cancel any NFS connections.
The mtab file could be locked for writing (access blocked by the umount of the USB drive), but the reboot should have fixed that.
The same should be true for the stale NFS handle.
A copy of the contents of /etc/fstab would be nice too, as well as a summary of the drives (XP & other) you are talking about.
What you could try, is to boot up interactively or in a special mode without mounting any drives (perhaps even from a LiveCD).