If this is normal SQL Server License (not some kind of OEM) - I think you can use your SQL Server 2008 on virtual machine (according to this article on TechNet).
Microsoft licensing mechanisms are complicated, I think you should call to Microsoft and ask about this for reliable information.
SQL Server registration data is stored in registry, here:
SOFTWARE\Microsoft\Microsoft SQL Server\80\Registration
Technically - you can backup that registry part and restore it on virtual machine, however I'm not sure if this is not license violation. Maybe you should call to Microsoft and ask if you can use product if you lost your key.