I can only tell you what I guess will happen in my case. It's a little bit of information and a bit of guess work.
the old girl bought me a compaq core2 duo laptop from PC World. ( My mate works there and advised her on the best deal ). PC World don't give out vouchers as such, but the receipt is proof of an upgrade disk.
Because it is a HP / Compaq model. HP are or were going to ship there own upgrade disk to match the computer purchased. This would mean that not only should we get the Vista home premium upgrade but it should also contain all the vista drivers we need for the machine.
So what I am saying and what I think may happen for those with the HP / Compaq badge label is
The disk you get, may be tied down to the machine you have bought. It may contain a trigger that needs to recognise the HP / Compaq installation before it will load on to the system.