Can be tricky - many boot managers can easily handle two
partitions but fall over when asked to swap physical
drives at boot time, esprcially when they are different drive types - like IDE and SATA. If you are using the same drive
channel for your two drives you might have better luck.
However, there are numerous traps you could fall into. For instance, installing Win 7
after XP is usually successful, because Win7 came after XP, and can set up a dual boot in the boot manager for Win7, called BCD.
But doing things the other way, well. XP knows nothing about Win7, and will either completely overwrite BCD with its own booter (boot.ini, ntldr.exe) rendering Win7 inaccessible, or will be unable to write a boot sector at all, leaving you with no way to access XP, since only Win7 continues to load.
Be prepared for trouble. Maybe someone who has done this can give full instructions, which are likely to go along the lines of (1) remove Win7 drive. (2) install XP on sole drive. (3) re-connect Win7 drive and use EASYBCD to insert a loading line for XP in the Win7 boot loader.
Good luck - you might need it.
http://www.sevenforums.com/tutorials/8057-dual-boot-installation-windows-7-xp.html