Quote: Originally Posted by marwenpix
I am a total newby --how does my computer know if these disks i have - have been installed before?
If I understand what you're asking: your computer doesn't know.
However, the latest three versions of Windows (XP, Vista, and 7) all require activation. Microsoft maintains a record of whether a license key has been used.
That's for retail and OEM licenses. I'm unfamilair with corporate licenses, but I believ that they're handled differently.