I'd try Bochs now, but I can't make heads or tails of the documentation.
Just for chuckles, I installed Crush! Deluxe in Windows 98SE in Virtualbox (which does not like Win9x guests
at all) and that gives quite a smooth slowdown. It only works for that purpose if hardware virtualisation (specifically VT-x/AMD-v) is disabled. Disabling hardware virtualisation doesn't seem to work for Virtual PC though, but then it doesn't say what exactly is being disabled.