Yes, have vista on your computer before hooking up the XP drive. If you do that Vista will ignore the XP installation so it will not boot. You could then wipe the drive and reformat it as a second drive in the computer. You could also transfer data from the XP drive to the vista drive. You may have to take ownership of some files and folders to do this.http://support.microsoft.com/kb/308421
You can take ownership of a folder in XP using the steps that are listed below.
1. Log on with an administrator account.
2. Open Windows Explorer.
3. Open the Properties window for the object and select the Security tab.
4. Click the Advanced button.
5. Click the Owner tab. The current owner of the resource will be listed.
6. From the list, select the user you want to assign ownership to.
7. Click OK.
8. Click OK to save your changes.
For Vista if you need ithttp://www.mydigital...-menu-in-vista/
I would not suggest trying to load a bootloader so you can dual boot the 2 OS's. The XP system was probably installed on a completely different MB and that would cause all kinds of problems getting it to bootup.