Check out vmware its probably the best to use with windows, and vmware fusion for the mac, just my opinion others will say different im sure.
Microsoft Virtual PC, Completely free
Code:http://www.microsoft.com/downloads/details.aspx?FamilyID=04d26402-3199-48a3-afa2-2dc0b40a73b6&displaylang=en
i use parallel on mac. works for me. havent tried vmware yet, will give it go as so many are liking it.