Used to have Vista Home Premium on my PC (no install disks) but after several Windows 7 installs Vista got overwritten, so I no longer have any 'proof of Vista' ownership. MS is offering 50% discounts on upgrade preorders ($99 for Windows 7 Pro), but I suspect I am SOL (i.e. will have to buy the full version at $300)? Will the Windows 7 upgrade work on my PC as it currently stands? (build 7100 64 bit). Comments!