If you buy windows 7 obviously you have your own disks in the deal.
Iffy disks are not within the province of this forum.
How busted is the 'old' computer?
There is the possibility that if the HD inside didn't suffer physically, it could be used straight away in any suitably specified new computer.
You would almost certainly have to re-activate W 7.
The only hurdle left would be sudden need of the OS disk for repair of some or other file, and a borrowed W-7 Premium should work for that type of activity