Logic, Logic, and Logic! Version5 runs under XP, but be forewarned, Apple bought Emagic, and has pulled the plug on all PC versions on October 1st. You can get the Digi001 for about $700. Cool Edit Pro will do 4 tracks, i'm not sure what it costs, though.
Pro-Tools Free is really the only free multi-tracker. Goldwave is another, it's shareware and does 4 tracks, and has no DirectX plug-in support. Cubase is probally best in it's price range, and uses VST and DirectX Plugins, which is cool. I personally never liked Cakewalk, although I know quite a few people who use it faithfully. It just feels cheezy to me, the plug-ins are lame, and I really don't like it's use of the file system.
My personal preference is Logic Audio Platinum, and the Waves Gold Bundle. This is what I use on a dedicated PIII 933 Rig at home. Plug-ins are great, the interface is stable, and rocks! The only weak point to Logic is punching in & out. What kind of Audio Hardware are you going to be running? Nothing is gonna beat the Audio Editing capabilities of Pro-Tools, once you've used it, you're hooked.
The studio I work at is still running a Nu-Bus TDM based 16 bit version of Pro-Tools (version 4.1) on a trusty old Power Computing (Clone) Power-Tower 120Mhz machine. Still does 32 tracks, just like the day it was built.