1. Backup all important data on Windows 7.
2. Install Windows XP into 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 correct drive for Windows XP
Manage Bootloader > Reinstall the Vista Bootloader > Write MBR
6. Restart 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 Windows 7 system drive to Windows XP system drive (enable "Show System Files" in folder options first).
9. Restart computer.
You should be able to boot into both Windows XP and Windows 7.