Writing often gives a false impression -- but here goes --It's NOT meant to belittle your intelligence or whatever -- but sometimes (and I've had plenty of these too) that a seemingly innocent remark in one place is totally mis-understood by others and then ......
However the whole point of Virtual machines is that a Virtual machine is essentially its own IDENTITY and totally isolated from the HOST. This process is known as "Virtual Machine Isolation".
Now sometimes the VM DOES have to make use of the HOST'S hardware and share facilities with the HOST and there are many mechanisms around for improving the performance of VM's - especially Virtual Servers - but software entries like the Registry should be totally isolated from th HOST or even other VM's.
I wouldn't worry about a Virtual Machine corrupting the HOST's registry -- UNLESS you get a virus and the VM can actually read and decode the host's registry file.
Remember that VM's are just as susceptible to viruses as the main machine.
I suspect though corruption of the HOST registry by the VM is probably only likely to happen once in the probable lifetime left for the Sun.