Hi, welcome to TSF.
Yes, you can do that. The easiest way is to change the boot order in BIOS, depending on what OS you want to run.
Or - you can set the "old" XP disc as #1 in boot priority - then edit the boot.ini file. There are other more complicated ways (of course).
But - XP might not want to boot because of the new hardware it lacks drivers for. Then you should perform an XP repair install:
http://www.michaelstevenstech.com/XPrepairinstall.htm