
Am currently building a new PC (Have case, PSU and RAM so far) and in all my budgeting I didn't think I needed to include Windows as knew I had 2 spare keys.
Unfortunately....my only Windows 7 key (Which was an upgrade DVD for my Asus laptop, from Vista 64Bit) is currently installed on my laptop and I have 2 spare VISTA keys. I must have had a mental block as just had it in my head that Windows 7 will be fine with them. Of course it's not

As I have doubled the budget for my PC I am reluctant and bloody annoyed that I will have to pay another £80 for another Win 7 key.
Can anyone shed some light on my best options here?
Currently only have laptop with Win 7 64Bit once new PC built, the new PC will be used for XBMC, Gaming (BF3), uTorrent plugged into my TV via HDMI and the laptop will simply be used for browsing if I have a film on the TV.
Option A: Buy another Windows 7
Option B: Use Windows 8 on PC as free but am very concerned about compatibility here.
Option C: (And very hard for me to do as will need the laptop up and running) Use the Windows 7 key from my laptop on the PC and install Vista on the laptop.
What's the best way of doing this? Is very annoying having 2 useless Vista keys!