1) You're on a P4 -- why Win2k? XP SP3 is actually faster and uses less RAM. (much like 7 vs. Vista, or XP vs XP SP2, destroying the idea that new releases just add bloat)
2) You're on a P4, unless that's a P4D, multithreaded will be WORSE performing than single threaded since there's nothing to run those other processes on. NOT that javascript runs in it's own thread in any browser. Even if it has hyperthreading, that extra pretend core is best left for OS level stuff instead of applications.
3) CommodoreJohn hit it on the head... IE6? <miz>REALLY? REALLY?!? REALLY...</miz>
NOT that firefuxxors is going to be an improvement on said platform -- given it's allegedly fast javascript is basically neutered by it's piss poor slow element rendering. (much less the memory leak hell it's ALWAYS had -- and that they have yet to even admit is a problem). But then I'm probably the only web developer on the planet with unkind words for that fat bloated slow buggy steaming pile of garbage that really is little more than Nyetscape 4's sweetly retarded cousin.
Chrome has the fastest implementation of most things, but the browser's UI is like a trip in the wayback machine to IE3 (Safari being much the same) -- It's why I'm an Opera guy -- hell, I run the latest Opera on win98 using KernelEx.
Though I suggest switching it to the 'native' skin, and sticking with Opera. For some reason the UI intimidates new users -- some people even have the balls to call it bloated... funny since the distro and executable isn't significantly bigger than FF or Chrome, and yet it has 99% of the stuff people add 'extensions' to those browsers to do AND a fully featured mail client in it... That's not bloat.
... and I don't care how good a virus scanner you're running, in terms of using the Internet IE, even IE9, is basically painting a bullseye on your backside and running around a prison yelling "rape me, rape me". You use IE for anything more than windows update, you get what's coming to you.
Oh, and you could always just turn Javascript off. Using Opera I selectively disable javascript on a number of websites when on my netbook since these days people are slapping endless amounts of javascript on websites for christmas only knows what! Certainly nothing useful... see the idiotic nonsense of javascript frameworks like jquery or mootools.
Ranks right up there with the other idiotic nonsense of things like CSS frameworks (that defeat the point of using CSS), people sleazing out HTML 3.2 and slapping a 4 tranny doctype on it, and HTML5 (which basically undoes the past decade of progress in web development rolling the clock back to HTML 3.2).