That's the idea but remember.. It might end up such way that you can't go
back to XP and the Vista won't boot either in the worst case.
A proper way to approach this operation is to buy hardware that allows to
connect the laptop hdd to desktop computer. This can cost from $3 (internal
converter) to $30 (external usb model). Then backup the laptop drive
contents 1:1.
It takes some doing but if thing go bad (and they *do*, read the
newsgroups), you'll be wanting the XP back as it were.