HiFlyer.
Not sure if this applies to you but I purchased a desktop off dell last year and got XP (with install disk). We then got a laptop off Dell which came with Vista (unfortunately) for my wife. We just used the original XP disk to zap Vista and put XP on.
The Dell OEM license covers this so you would be safe enough to do.
Note in this case it's the fact that our XP disk is covered by the fact that we're installing it on another machine by the same vendor!
The install process is easy enough (you just have to make sure that you can boot off cd which isn't always enabled....go into your BIOS to activate this)
You definitly want to get rid of Vista....the first service pack is just fixing known bugs. It will be some time before they address the performance issues in vista!