One of the new features of today's pests is to disable the antivirus or such. Try a web based scan like Housecall and see if it detects it.
While cleaning up my XP Startup, I found the following file: sysxp.exe.
When I search Google for this file, all the results I get refer to W32.Beagle.AB@mm - sysxp.exe. I've run McAfee's removal tool, but it doesn't locate any infected files.