I've never understood how the Serials thing works. And I've had all kinds of hassles with it over the years with different O/S's and upgrades and second hand machines and broken machine needing repair and reinstalls and whatever... Is there a clear outline of it somewhere? Say I have a machine at home and I install Windows on it. Then I take that serial over to another machine at home and I try to make another installation and use that serial again it won't work, will it? I don't think it will. Now if that first machine crashes - say system disk crash for instance, or mobo failure - then I have to rebuild that machine and install Windows. Now this time it will accept the serial, won't it? Or won't it? I think it will. I think I've done it in the past. I've done nearly everything over time without ever thinking about it. But how does MS know when I try to put on a second machine that it is already on one machine (that might be switched off, might not have been used for days, weeks) - and then knows in the other case that it is not on any other machine? I am obviously wildly confused about the whole thing. So how can I get a clear knowledge?