1.) Did the drive survive? If not then the data's lost and you''re looking at a fresh start. Hopefully you had backups if that's the case.

2.) Was that a pre-installed copy of Windows XP? If so, you cannot (legally and usually technically) transfer it to another computer and expect it to work. You can connect the drive to the new computer to copy off your data, but not run that copy of XP.

3.) If it was a retail version of XP, you can connect it and see if it's bootable. If not, you'll have to perform a repair installation on it.

4.) Assuming you can run XP off that drive on the new machine (see above), the simplest dual-boot would be to use your system BIOS to switch between drives at boot time. Otherwise you'll need a repair install to adjust the new drive to be the primary, and then a repai of Vista to make it bootable again. (There are complications with adding XP to a Vista machine.)

5.) Yes, if you can run XP on the new computer you could wipe the new drive and use it as storage. Personally, though, I bet that's an OEM copy of XP, meaning Vista is your only option.

John