Windows 8 Upgrade offer question urgent!


New Member
Hi guys!
I'm hoping you might be able to help me!
I have just bought a new pc. No hard drive. im going to be using a hard drive out of my mac mini that has vista on the boot camp. Im going to take the hard drive out wipe it put it in the new computer and then reinstall vista with the hope of upgrading to windows 8 for their £25 offer. prob is offer finishes tonight and I'm trying to find out if i can buy the license at £25 just now and get the license key and then next week when my computer arrives and i've transferred the hard drive can i download windows 8 trail from microsoft and then activate it with my key or does it need to be the version that that finishes tonight?

thanks in advance!