vista and xp o/s on same computer

I just bought a new PC running Vista. I knew some of the programs and files from my old machine running XP would not run under Vista so I added an external hard drive to the new PC, installed XP onto that drive and then cloned the old PC’s hard drive onto the external drive, but I have just discovered that I cannot boot from an external drive so Can’t run XP from it. The new PC ‘s hard drive is 500GB but actually consists of 2×250 drives. Can I separate these two drives so that I can have different O/S on each?