I ain't worried about legality of Upgrade installs as I have a licensed retail boxed version of Vista HP. When I'd first installed Win7 into my rig, it detected Vista and I was able to install easily enough. But when I picked up an SSD (spur of the moment last minute deal), I had to install it fresh. W/o an existing OS, I couldn't upgrade, so I took the long route and installed 64bit Vista HP (retail) into the SSD. I did not have to activate it as I'd then install Win7 as a clean install, everything went as I'd expected. I believe that anyone with a legit retail copy of a previous supported OS would have no problem installing Win7.
