Virtual Machine is the way to go, along with the time-bombed SharePoint 2007 dev environment VM. This VM expires in 30 days, but if you are an MSDN or TechNet subscriber then you can activate it for another 4 or 5 months.
Use Virtual PC or Virtual Server, they're both free and they read VHDs.
The "run MOSS on XP/Vista/Win7" from Bamboo is not a bad solution, if you're just looking to learn. But the virtual machine is the best way to go.