Buy a technet subscription if you have several computers. You will get 10 keys for every OS microsoft has, XP, Vista, Windows 7, not to mention server, and office, and every other MS software ever made.
You get 10 keys for each version, basc, home, pro, ultimate, and each key will work for 10 activations.
You can buy the subscription for $349 if you wish to download your .isos, or for $599 if you want the actual discs shipped to you.
I got my subscription free, I know of 2 ways to get it free for a year, a little google skills will show you how, or you can just buy it. Its well worth the money if you have multiple PCS you cant beat it.
The keys you get are full retail keys. They never expire.
The PC in your sig probably wont run Windows 7 very well.