you can use the autoruns utility (freeware) to disable any unneeded startup programs (I bet you've missed a few)
try using KMeleon instead of firefox (it is it's lightweight cousin)
boot into safemode and surf/wp from there if it's really bad.
One last thing, I have never tried this but Blackbox is supposed to be an extremely lightweight and fast windows replacement shell, please let us know how it goes if you try this:
http://www.bb4win.org/news.php
review:
http://shell-shocked.org/article.php?id=253
from the review:
"To quote the manual: [BlackBox]'s not meant to be Eye Candy ... , nor is it meant to be the most fully featured shell out there. It is just meant to be fast. And that it is I can assure you. It loads in no time. The menu pops up the same instant you click (well, it really doesn't of course, but it feels that way). Switching to a new style takes only a few milliseconds even on my dinosaur computer. I can in fact notice a remarkable overall speed increase when running BlackBox, almost like a slight processor overclocking. I don't have any benchmark figures to back this statement up, but even if the shell doesn't actually make your computer faster (probably just a matter of graphic rendering speeds) it's nonetheless pleasant. Even if it's really just an illusion of speed, it's a convincing one.
As far as resource usage goes BlackBox is much lighter than Explorer, but I couldn't detect any notable difference between BlackBox and LiteStep. Both shells utilize around 7-8MB of memory, but it must be added that BlackBox sure feels a lot faster and more responsive than LiteStep, which possibly can be ascribed to BlackBox's bitmap-less approach. One would think that it also might have something to do with BlackBox being a port from Linux (since Linux is always faster and better than Windows... right?), but actually the Windows version doesn't have the same codebase at all except for the rendering engine.
What about stability then? Well, right out of the box (no pun intended) BlackBox runs like clockwork."