VMWare starts at about $300 USD for the workstation edition, all the way up to several thousand for "mainframe class" versions. With this product, you could build a Windows machine on top of a Linux workstation, or a Linux workstation on top of Windows. You could essentially run any OS that would normally run on Intel hardware, if I understand correctly...
CygWin is absolutely free - the catch is that you have to download it. You can't buy a CD for it, that I know of.
There is also
U/Win, and I briefly tried that one out. Beware though, that if you install it on a workstation that's on a large domain, it'll try to mirror user folders like it were a server. It was pretty obtrusive, but I have't tried it on a standalone machine...
Those are the three main choices that I know of...