1 computer Sophos? (windows 7)

Hi,

easy: how do I get Sohpos(only really need anti-virus part/endpoint) for windows 7 for one computer?

detailed:  I have a mac mini and I bootcamp to windows7 on occasion.  I do have sophos for the mac os X partition and it's free.  I also have sophos anti-viris on my windows7 partition but it's only a trial version.  So,  after a few weeks(month? or two) Sohpos on my windows 7 partition will stop updating (a shield icon with an x on the lower right side in the system tray).   I really like Sohpos, so how would I get out of the trail version of it for this one computer.

difficult: I am getting a new computer shortly (fragbox) it will have windows 7 on it.  So I would love to have Sophos on that as well, well alright I'll prob take the bootcamp partition off of my mac mini and just use the new fragbox for windows7.  So really only need Sophos for the fragbox.  Then again I might keep the bootcamp partition as well and need Sohpos for that one as well.

well I did look up prices.. said 3 users.. for 1 year price.. this what I need?  user/computer (confusion)?

thanks in Advance..

Steve