From recent posts it seems that it's WinXP that has problems with dual cores. The latest build of Windows (in beta testing) is going to be a huge resource hog of an OS, and totally unacceptable to me (and probably most) [EDIT: By this I mean "no use waiting for the next Windows, it won't help much"], so that leaves only Win2k, which is reported to have no problems with multiple processors.
It seems the problem is the OS, not the hardware.