If you really don't want vista you can change it to xp, I did on 2 new machines.? XP Pro with 1 and XP Media on the other.? Sometimes or I'm sure anytime you try to reuse the cd serial number it is found to be in use already.? I reused XP PRO on an old PC and couldn't update anything and it eventually it wouldn't run right with newer programs and got slower and slower.? Good Luck
There are ways to update with a serial key in use, but it takes some searching of the internet and is probably not entirely legal. However, if you own the XP software (and have wiped it off your old computer) then you could always call Microsoft and have them give you a new CD Key. I believe they will do so up to 5 times (for situations when you need to replace your hard drive, format, etc.).