|Firstly, if Vista is anything like XP it is tied intricately to the hardware environment it was installed in. Therefore the Vista operating system that you have which was installed in a Dell machine will not boot up in your hardware.|
Second, Just attaching a harddrive with a pre-installed operating system as a secondary drive does not automatically trigger a dualboot menu.
If you wish to try booting up Vista then you can do so in one of two ways. Remove your XP drive and leave only the Vista drive. Then see what happens when you try to boot.
The other method would be to leave both drives attached as they are and go into BIOS to reset boot order so that the Vista drive is ahead of the XP.
In either case I will bet Vista will spit the dummy.
My recommendation. If there are files you want off the Vista drive then transfer them to your disk. Format the vista drive and if you want a dualboot then get your own version of Vista and install it. The Dell version, even if you have the Dell install disk probably won't even instal in non-dell environment.
Goin' Fishin' (Some day)