The quickest way to get started with virtualization is to download free pre-built virtual machines. These VMs already have an operating system installed and usually already have a application installed. For example, you can download Microsoft evaluation virtual machines (actually VHD files) with some of their latest enterprise applications (like Exchange Server 2007 or Windows Server 2008).
Another excellent source with hundreds of VMs available for download is the VMware Virtual Appliance Marketplace. At this site, companies and individuals have contributed VMs that anyone can download – both for commercial evaluation of software or just for fun. You will, of course, need a virtualization product. I recommend the free VMware Server or Microsoft Virtual Server & PC. To download these, see Virtualization FAQ #44.