1) for simplicity start with vmware player as it's FREE.
2) If you need a bit more sophistication such as "Vm cloning", "Roll back", (i.e the vm doesn't make permanent changes to your Disks -- at the end of the session the vm is restored to initial state ),or run as a server (VM's run in the Background and can be accessed even if the user doesn't have an account on the Host machine) -- then get vmware workstation -- paid for software.
Esxi is also very good and Free -- but 1) It's a bit fussy on what hardware it will run on -- look at Esxi White Box configurations, and remember you will need a separate machine to control the vm's from -- for example run esxi on a workstation -- but you will need to connect a laptop or other machine to it for acessing the VM's themselves.
Vmware Esxi is a TINY OS so the vm's run almost at native speed and for a lot of peripherals you can use PCI passthru which allows the VM to access the actual PCI hardware which normal VM's don't give you access to. http://www.vm-help.com//esx40i/esx40_whitebox_HCL.php
Remember also that your VM in most situations will require a separate Windows licence --assuming you are running Windows VM's. You can "Clone" VM's with the same licence so long as you don't run them concurrently. So no prob testing something, trashing the VM and powering on a new one.