I agree with Computer_Commando as for I have run into issues with multicore & Multiple CPU's and older OS, and in Virtual PC 2007, which is what I use, all older OS run correctly within. ( But games for Win 98SE wont work too well with virtual GPU which is like a low end 2MB videocard simulated. )
I ran into an issue where Windows 98 although ran on a multi-cpu system, would not address the extra CPU a while back when I acquired a dual-cpu motherboard. I would assume that multi-core CPUs would probably fall into the same category. I ended up having to make the upgrade to XP to get use of the power of both CPU's. I then went with the first version of Virtual PC... might have been 2004 which cost money back then, but has since become free from microsoft in version 2007