yes its possible; the concept is called dual booting. If you perform a google search and use "dual booting win xp" tons of info will be presented.
However dual booting on the same drive is not a stable thing IMHO. if either of the OS's crash and wipe out a partition or the MBR (master boot record) then you will lose both OS's and data. That really hurts!
you would be better served to buy another hard drive and install vista on that one. You can buy new Western Digital 200 gig Sata or IDE drives for about $60.00 on ebay.
for a very affordable price you can gain alot more independant stability!
dotn hesitate to ask more questions; should you choose to proceed anyway; just holler and we can assist you with the steps involved. but be warned, adding more partitions (dual booting requires another partition) on an existing boot drive that has Win XP installed means using partition software (there are free ones) but using partition modifying software can be tricky for first time users, you may well frag your existing boot OS while learning the ropes ? but sometimes the adventure can be fun too
