As I mentioned in the Computer Love post below, I bought the Windows 7 family pack. It's on sale right now at Amazon for "only" $99.99
Microsoft Windows 7 Home Premium Upgrade Family Pack (3-User)
As I said, this is only an upgrade, not full version but the price is the lowest I can find, right now. The price seems to change almost daily... Obviously the reason I'm posting this is to drive a little Amazon Affiliate traffic, but it is a good deal...and I only found it because I was surfing for some other stuff...I'm such a shill...
UPDATE: It's back up to $123.99. It's interesting to look at historical Amazon pricing with camelcamelcamel.
The power supply I used is on sale at Newegg as well, 10% off using a promo code (An10psu ends 3/27, and I don't make any money off of it...)