Panic! On my dual boot pc, a failed linux upgrade had wiped the linux boot partion.
The Windows partitions is still there, but unbootable: the boot process was struck at a grub error
error: no such partition.
grub rescue >
The MRB had to be rewritten but I had no Windows install CD ready at hand.
By chance, I had a GParted Live CD in the desk drawer.
I've booted it and luckily it had the ms-sys command on board: ms-sys just rewrites a Windows MBR on the HD.
# ms-sys -m /dev/hda
Windows was back up and running.