A Windows 7 install disc/dvd (not disk) can be used on any PC you want, if the key is a retail key it can be used on any PC you want, as long as the reatail key that came with that disc is only used and activated on 1 PC, it can not be used on more then 1 PC at a time.
An OEM key is tied to the motherboard it is 1st used on, it can be used over and over again as many times as you want on that motherboard.
If that motherboard dies, you are supposed to buy another key.
But if that happens you can always call Microsoft, explain what happened, and see if they will issue you another key.
A Windows 7 install disc/dvd is not tied to any PC, it`s just an install disc.
Branded install discs (HP, Dell) etc are meant to be used on the PC they are sold with for automatic activation purposes.
PC manufacturers stopped giving out discs with their PC`s a long time ago.
Not sure if they came with a Vista PC as I bought Vista retail (Twice)
But seriously, I Loved Vista :)