You said that you reinstalled Windows. Did you do a repair from the XP disc or did you reformat and re-install? There are two things that I can think of that may cause this. One, is that your USB driver either didn't install or is corrupt. Try another USB device and see what happens. The other is a registry error. Your registry may have "issues" from the previous install but that's a slim chance if you did a complete reformat.
Dan