I have an old fujitsu seimens laptop running firefox with AVG (free) and firefox takes ages to start on that.

For my three other machines - Advent desktop, HP laptop & Asus netbook I bought Kapersky 2010 because of the great reviews, but found it to be fartoo intrusive and slowed everything down - since upgrading to windows 7, I have installed panda global security on all three and uninstalled IE. I am now using chrome on the asus and firefox on the others. The fastest by far is chrome/panda on the asus!