Is your copy of XP, OEM or Retail? If it's retail you won't need a copy of Vista as you can install it (the XP you have) onto as many PC's as you want. If it's OEM you need a new copy of an OS, whether it be XP again, or Vista. OEM copies can only be installed on one PC.
What do you mean about USB drive? If your talking about a USB harddrive then I would get a new internal one.