I know of no reason why you would have to reinstall Vista just because you
attached an external hard drive. Your mobo does not have an external eSATA
port or an external SATA enclosure would be the way to go. Otherwise buy an
extenal usb hard drive enclosure and something like a Seagate 7200.10 250GB
or 500GB drive and assemble it (easy). There should be nothing else you
have to do but power it up and plug it into a usb port.

I recommend Newegg for the components or for a pre-assembled drive.