There is nothing at all wrong with Firefox.. great browser, built-in popup blocker, tabbed browsing.. that being said, sp2 for xp implements a lot of those same features in IE.. I am just not one for tied in to the OS apps such as the way Microsoft presents them..
for the free tools, i only use Spybot: Search and Destroy -
http://security.kolla.de - for spyware based problems/popups/etc..
there is also adaware (i think that's the correct name) and a few others.. but i am not very familiar with any of those..