I'm not positive if they're still doing the deal ( i thought it ended in january but i saw stuff about it recently) but the cheapest way would be to be a student/have somebody who is a student buy it for you. You can get it for 30 dollars.
http://www.microsoft.com/student/en/...windows-7.aspx
i think you can also get and/or burn the upgrade on a disc but I'm not positive..
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