If you have a dual-core CPU, Windows runs just as well in a virtual machine
as it does natively in dual-boot mode using Win4Lin Pro version 5.

http://win4lin.net/content/

Single core CPU OK as well but does not quite run at native speed!

I have been using Win4Lin products for several years and the Pro series
since the early beta (pre version 1.0) releases. It just keeps getting better!