windows OS question

hey guys, quick question:

i am currently running win7 64-bit on a laptop, i want to partition a piece of the hard drive and put windows XP Home Edition on it (32bit), my question is: can i buy the cheaper upgrade version of xp, and install it on that empty partition since i already have a genuine copy of a windows OS (win7)

or will i have to buy the full version of winXP home edition 32bit?

thanks

ok thanks guys! ill give that virtual pc program a try