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.
"bootrec.exe /nt60 C:\
" (default position of your Windows partition is C:\).
After that, type bootrec.exe /fixmbr bootrec.exe /fixboot bootrec.exe /rebuildbc
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.