In that case, you should ask your computer manufacturer (I assume it's not a self-built system?) to ship you a restore disc. They'll charge you for it, but it should be cheaper than buying a new license. I believe copies of Windows on restore partitions/disks come pre-activated for the manufacturer systems they're meant for.
Alternatively, you can try
Windows 8. I hear it's pretty stable in its current state, and it'll probably be free until MSFT releases Win 8. As a nice benefit, it's built on cutting edge tech, has superior performance to Win 7, etc.
And OEM = full version. They're actually meant for use by system builders who re-sell the computers they build, that's why they're discounted to $100 and whatnot. The Upgrade version will only install if you have an activated copy of Windows on your hard drive when you're trying to install the upgrade.
Also, if you happen to be in college, you may be able to get a free copy through the college's MSDNAA/DreamSpark subscription.