I run a few Win7s and several XPs. There are a few solutions that I use - there are programs that let you change the Key on a running machine.
Sometimes, you have to ring them up and listen to sale bs to get it re-registered, IF that instance requires it.
I have also used standard sysprep images from generic discs on ALL types of laptops, they work fine. Just download all the dirvers required for that make/model.
With XP, there are TIME LIMITS on how many times you can install the same Key, 1-3 for the first 3 months etc etc I don't think it is totally open,
but I've never done that, so I don't know. It's not difficult to still find cheap Keys.
As far as Win7 on 2 machines at once on a network - sometimes (rare) on a sysprep roll-out, I've had identical licenses on 2-3 machines, while doing the hardware
upgrade. Often it just gave me 30 days. It wasn't an issue, because the old PC was getting killed off anyway. Again, I used a program to just change the key.
If you have legal Keys, just reset them. I've never had an issue that I couldn't fix easily.
Note: In a couple cases, MS had a campaign to force laptop manufacturers from making generic drivers available for laptops, and you could only use the brand disk.
That was pretty easy to sort out as well, just get a list of the chipsets, then find similar drivers and try them out 1 at a time (Note keep an Base Image and reload
it each time) It rarely takes me 1-2 goes to find one that works FINE !!