Heres to the lightweight WMs!
larsenmetl: You can use any one of them and still use those apps. Firefox and Limewire aren’t KDE specific, anyway. I myself use K3B for burning CDs and even managed to fire up KDE’s control panel from under IceWM (by invoking kcontrol) when my keyboard repeat got screwed up for some reason.
Personally, I recommend IceWM. It’s got the Windows-like taskbar and alt-tab window switching. You’ll definitely want to learn how to edit the config files kept under ~/.icewm to customize it to your liking. One tweak I highly recommend is setting your taskbar to double-height. That command line helps keep your taskbar uncluttered with extraneous xterms.
I said I recommend IceWM, but try a whole bunch and see which one fits your fancy. The sheer variety can be a bit mind-boggling, especially to a former Windows user who was locked into one window manager (I know there are others, like Litestep, but seriously).