The trial version is your cheapest way to do it. The trial software runs for either 90 or 180 days, i forget which. At the end of that period, you can simply reinstall it.
The second option would be to invest in a technet subscription. This would cost you $199 for a year, but you get keys and software downloads for tons of stuff (all windows clients (xp, vista, 7) and all versions (standard, pro, ultimate), and you get office (2007/2010) and you get the server software (server 2003/2008/SQL/Exchange). It's an absolute bargain for the price.