This has probably been asked before, but I want to be 100% sure before buying: if I have a laptop with an OEM version of Windows Vista Home Premium installed, can I buy the Windows 7 professional upgrade and install it ? I don't care whether it will be an actual upgrade or a clean install (that would actually be even better), what I'm curious is whether this is actually legal (going from home to professional). 

And if this is possible, how does it work ? Do I have to have Vista installed when upgrading so that the installer can check that I actually do own a genuine copy or does it ask for the vista key (which I'm not sure I have, since it's an OEM version) ?