How to restore a computer to a previous Windows installation after you install Windows Vista
http://support.microsoft.com/kb/933168
I note this method of restoring boot sector
Step 5: Restore the boot sector for the previous Windows installation
Type one of the following commands at the command prompt, as appropriate for your situation.
Note D: represents the DVD drive in the following commands. If the DVD drive on the computer is represented by a different letter, such as E:, use that letter in the command.
• When the previous Windows installation was Microsoft Windows Server 2003, Microsoft Windows XP, or Microsoft Windows 2000
Type the following command, and then press ENTER:
D:\boot\bootsect /nt52 c:
• When the previous Windows installation was a different version of Windows Vista
Type the following command, and then press ENTER:
D:\boot\bootsect /nt60 c: