By the time you try to call Microsoft and go through that rigamarole, you might as well just buy a copy. Since someone built this computer for lexxy, there won't be a sticker on the case unless she put it there. I never put my stickers on my cases either - although I probably should. And since this guy put on a pirated version, there probably isn't a valid number anyway.
That will be fine, lexxy. mj2c has a good suggestion - put the validation code sticker on the case so you will never lose it.
If life gives you lemons, make lemonade. If life gives you tomatoes, make Bloody Marys.