Its simple. just install xp on a different partition and it will be done. i am using quadruple boot with linux and windows. i have dual boot xp on same hdd 160 gb (windows 7 and xp) and i have no issues regarding.
you might lose windows 7 bootloader if you install xp after windows 7. it can be recovered as :-
Click on "Repair Your Computer" to gain access to the System Recovery window after booting with win 7 dvd.
Now choose "Command Prompt" in order to run the desired utility which is called "bootsect.exe". Bootsect is located inside the boot folder so change your directory to boot.
Now run
"bootrec.exe /nt60 C:\" (default position of your Windows partition is C:\).
After that, type bootrec.exe /fixmbr bootrec.exe /fixboot bootrec.exe /rebuildbc
4. Restart your computer and your computer will surely run fine. Important note : If your directory is not C:/ , then you can type bootrec /ScanOs to get full list of Os installed on your computer and select accordingly.
My Computer
At a glance
Windows 7 + Windows Xp Pro + Ubuntu 10.04 + o...Intel C2D 2.0 GHz2.0 GBIntel Chipset 945GCT m2
Computer Manufacturer/Model Number
ECS Elite group
OS
Windows 7 + Windows Xp Pro + Ubuntu 10.04 + openSUSE 11.2
CPU
Intel C2D 2.0 GHz
Motherboard
ECS
Memory
2.0 GB
Graphics Card(s)
Intel Chipset 945GCT m2
Sound Card
Realtek
Monitor(s) Displays
Samsung LCD
Screen Resolution
1280*1024
Hard Drives
Samsung Sata - 80gb
Samsung Sata - 500gb
Hitachi - 160 gb
External - WD - 500 gb