Recommendations for running FreeBSD as a guest OS

[Bobby Walker]

I have a small network at work that, unfortunately, uses Windows 2003.   
I need a good mail server, but I do not have a budget for purchasing 
additional software.  Exchange requires too many hacks to configure a 
catchall email account, and Exchange and I do not get along very well.  
So, today while brainstorming, I thought why not run FreeBSD as a guest 
OS on the box.  Any suggestions for the best way of doing this?

Thanks in advance,
Bobby