" Can I upgrade XP to Vista and keep XP available in case I want to go back ? "
" Or upgrade XP to Vista on one HD partition only, so have both available ? "
You would have to partition your hard drive and put Vista on the partitioned drive. That’s if the drive has enough space.
You would then have the option to dual boot the computer. You would have a choice into which operating system you want for that session.
Have you checked to see if your computer is good enough to run the version of Vista you want to use?