A.)yes you can install windows xp on other partitions or other hard drive thats is dual booting, everytime you start up you have to choose which operating system you use vista or xp. other option is install virtual machine on your windows vista operating system then you install windows xp on it. just google it about virtual machine it is free download at microsoft. it means installing other operating system under your current operating system.
B.) maybe yes because your friends does it.