I just built a new system. It had been working for a few days now... but then 'The product key you typed cannot be used to activate Windows on this computer.". But I bought this... a retail copy, even. I had this problem with my first gaming rig a couple years ago where I actually had to call MS support and talk to them. problem: I'm down to around 10min call time on my phone so I can't phone in. What can I do (aside from using a crack or someone's key)? No, this isn't an upgrade disc. It's full Windows Ultimate.