Re: acer laptop
I had exactly the same issue!
You'll have to do a full format and you'll need a copy of a Win7 disk. What I did was get a copy of Win7 from a torrent site and used the serial key on the laptop to install it.
Not sure exactly how the law stands on that method but AFAIK I haven't deprived MS of anything - the serial key is genuine.
Be warned though, there will be NO drivers on it so you'll need to either pre-download them on to a memory stick or ethernet the laptop to the modem/router. Get them all from the Acer site.