As said, multiple machines or at least multiple hard disks or partitions 
each with it's own OS and dual boot.
Or you could use an emulator.

If you have the resources and the $$$:
http://www.vmware.com/
http://www.connectix.com/products/vpc5w.html

Only needs the resources (don't know how good it is):
http://bochs.sourceforge.net/


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