I am having the exact same problems. I had a Linksys wireless g usb that the box and Windows Compatability website said was compatible and briefly it was running at 1900+/- kps per tech.msn.speedtest when first installed. However, it's a new PC and I ran the Windows Updates and ever since the fastest it will run is 185 or so KPS. My laptop is sitting right next to it and runs (with an older wireless g internal card) at 2000kps. I returned the linksys thinking it was a hardware issue, got a Trendnet tew-644ub (again the box and windows website says is x64 compatible), but its never got faster than 190kps...
NOVICE skill, please explain in a "For Dummys" manner... Thanks!