To sum up:
If you have a PPC Mac (G3, G4, G5) then EMULATION is necessary to run Windows, because Windows doesn't run on PPC hardware.
This emulation "translates" PPC code to Intel (x86) and vice versa, which takes time, which means... SLOOOOOOOWWWW.....
Example app: Virtual PC (version 7.x for the G5)
Experience shows that Windows 2000 Pro running on VPC 7.x runs okay.
If you own an Intel (x86) Mac (Core Duo, Core 2 Duo, Xeon) then VIRTUALISATION is all that is needed to run Windows. As the CPU is "known" to Windows, no performance is lost because no CPU-"translation" is needed.
Grfx cards, etc are still emulated though, therefore don't expect "native" 3D grfx performance.
Example apps: Parallels, VMware, VirtualBox.
All versions of Windows run very well in these virtualisation apps.