I have installed the free Avast Antivirus on my Windows 7 partition, whereas the Windows XP has the new free AVG 2011. I was using AVG (previous versions) till date on all my systems and OSes. But the latest version of the free AVG seems to be somewhat unstable. So removed it from my Windows 7 and installed Avast and since then I am quite happy with it. I am planning to install Avast on my Windows XP too.