1. Backup all important data on Windows 7.
2. Install Windows XP into a separate partition.
3. Boot into Windows XP. (You won’t be able to boot into Windows 7 at this point).
4. Download and install EasyBCD. EasyBCD requires Microsoft .NET Framework Version 2.0.
5. Run EasyBCD:
Add/Remove entries > Add Windows XP
Change Settings > Select the correct drive for Windows XP
Manage Bootloader > Reinstall the Vista Bootloader > Write MBR

6. Restart the computer.
7. Boot into Windows 7 (You won’t be able to boot into Windows XP at this point).
8. Copy boot.ini and ntldr from the Windows 7 system drive to the Windows XP system drive (enable “Show System Files” in folder options first).
9. Restart the computer.
You should be able to boot into both Windows XP and Windows 7.