you can install it without a key and then put it in when you have one. You get 30 days of "time to put your key in" windows coming up when you turn on the PC, then after that it just says "this copy of windows is not genuine".
I get lazy when I re-install.

Anyways, you can get yourself an ISO and burn it to a DL DVD and bam. Put your key in when you got it.