I'm getting a headache just looking at the available options:

Windows 7 Home Premium Full
      
Windows 7 Professional Full
     
Windows 7 Ultimate Full
   
Windows 7 Home Premium Upgrade
        
Windows 7 Professional Upgrade
      
Windows 7 Ultimate Upgrade
      
Windows 7 Home Premium Upgrade Family Pack

Microsoft Windows 7 Home Premium 32/64-bit (Upgrade from Windows 7 Starter
    
Windows 7 Professional 32/64-bit (Upgrade from Windows 7 Home Premium)
    
Windows 7 Ultimate 32/64-bit (Upgrade from Windows 7 Home Premium)

If you are ready to take the plunge, be sure to download the windows 7 adviser to see if your setup is worthy

http://www.microsoft.com/downloads/details.aspx?FamilyID=1b544e90-7659-4bd9-9e51-2497c146af15&displaylang=en

Microsoft is now offering it's free Anti-Virus offering, Microsoft Security Essentials at http://www.microsoft.com/security_essentials/default.aspx

Iobit is offering a free license of IObit Security 360™ PRO at http://db.iobit.com/license-free/win7-special-offer.php

AVG Antivirus free version 9 at http://download.cnet.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10320142.html?tag=mncol;txt