Update MBR in WinXP:
1. Restart your computer with the Windows XP Setup disk in the CDROM drive.
2. Press the R key to start the Recovery Console.
3. When Recovery Console starts, it will prompt you to enter a number corresponding to the Windows XP installation that you need to repair. In most cases, you'll enter "1" (which will be the only choice). If you press ENTER without typing a number, Recovery Console will quit and restart your computer.
4. Enter your Administrator password. If you don't enter the correct password, you cannot continue.
5. At the Recovery Console command prompt, type fixmbr and then verify that you want to proceed.
Update MBR in Vista/Win7:
1 Insert Win 7 installation DVD and boot from DVD drive. (or Win7/Vista Repair CD)
2 Choose your default "Language", "Time", and "keyboard Input" on the first window and click next.
3 You're now presented with 3 choices. Click on "Repair Your Computer" to gain access to the System Recovery window. Now choose "Command Prompt"
4 Now run "bootrec.exe /fixmbr"