I like my Windows 7 64bit system that's running avast! V5.0.239 beta2
My old XP Pro system is still working great as well.
E5200 2.5GHZ, 4GB RAM, 320GB HD, Windows 7 Home Premium 64bit, avast! V9.0 Free, IE10
P4 2.8GHZ, 1.5GB RAM, 40GB HD, XP Pro SP3 32bit, avast! V9.0 Free, Google Chrome
with hpHosts, MVPS HOSTS files, SpeedFan, WinPatrol PLUS