Comment Vmware Player or Virtual Box (Score 1) 3
Vmware Player or Virtual Box are both good starting points on a Windows host, they are free and relatively easy to get started with and use.
Reliability wise they are fine but Virtual Box seems a bit slower and doesn't really give you an upgrade path like VMWares product line.
Unless you are looking to eventually upgrading to a MS Server 2008 based Hyper-V system I wouldn't bother starting off with Microsoft's Virtual-PC/XP-Mode product.
I have no experience with Xen, so can't comment on it, but I'm sure someones else will.