I did a very silly thing this morning. I deleted my Ubuntu, and Vista partitions without fixing XP’s MBR. This basically leaves grub in rescue mode because there is no boot partition. I have the Vista disk but it was unable to fix the MBR or so I thought.
To fix the XP MBR (from a vista downgrade or whatnot) do the following:
- Boot from the Vista DVD
- Select repair install when the screen shows up after the language settings.
- Then open the command prompt aka restore counsel:
Type the following: X:BootBootsect.exe /NT52 All
Note: X:/ is the drive letter where the Windows Vista media is located (the command prompt will show the letter).
After the computer restarts it should load XP’s MBR again 
.
If it doesn’t still work or you use Vista and have the Vista dvd then just use the repair startup tool 2-3 times.
