Method 1: Use MBRFix
MBRFix is a tool to fix Master Boot Record (MBR) on harddisks. It also works for Windows Vista.
For old PCs, just use Windows XP install disk to repair it by use fixboot and fixmbr commands.
Method 2: Use Window XP/Vista Install CD/DVD
For PCs with SATA disks, the windows XP install disk can not fix the boot problem. You need use Windows Vista install DVD to fix the Grub problem.
How to use Windows Vista install DVD to fix the boot problem?
1. Put the Windows Vista installation disc in the disc drive, and then start the computer (set to boot from CD in BIOS).
2. Press a key when you are prompted.
3. Select a language, a time, a currency, a keyboard or an input method, and then click Next.
4. Click Repair your computer.
5. Click the operating system that you want to repair (Vista in this case), and then click Next.
6. In the System Recovery Options dialog box, click Command Prompt.
7. Once in the command prompt, type exactly Bootrec.exe /FixMbr and then press ENTER. You will see "operation completed successfully."
8. Reboot and set BIOS to boot from the HDD again.
GRUB will be overwritten in step 7 and your old bootloader (windows xp, windows vista) will once again take control of loading your OS(s).