Once you've installed it you can go to add the virtualization role in Server manager ... and you'll find it isn't available. What you need to do is navigate to the Windows folder and find a sub-folder named WSV. In there are two patches, apply them both in either order, go back to Server manager and you can add the role.
Don't forget that you need to have a 64 bit machine, and it needs to support Intel VT or AMD-V (previously called Pacifica) technology. On most machines you need to enable virtualization in the BIOS and do a cold boot - you must power off and on simply letting the machine reset after changing the BIOS setting won't work.
Be aware that Server 2008 presents hardware which is different to Virtual PC and Virtual Server and if you're bringing over existing VHDs you may need to re-activate the OS
If you are UK based and would like to be on the IT-Pro Momentum programme please drop me a mail with the word "airframe" in the subject. If you're outside the UK I can try to link you up with the local person in your region.