Buying Windows Keys From used Pcs

[DM1000]

I always see broken or old computers on craiglist for cheap that still have Windows. Is it possible to buy these PCs and take the key of them and use them on a new PC build?

 

[AppleGwava]

Yes and no.Windows have the programmed habit to lock up when too many hardware changes are detected.Its is still down to luck though,usually you can call microsoft and ask them to authorise it

 

[misteriosly]

Depends on license.
Some licenses are OEM, which means when user install windows and registers it online, windows bind to some hardware serial Num, usually its motherboard s/n.
If license is non OEM you could use the license on different PC without a problem.
If you are looking at those Small form factor or other premade pc's like Dell or IBM or Fujitsu, they are all with OEM licenses.
Seems that they get OEM licenses cheaper, since they build a bunch of those premade PC's and most of them are Microsoft partners.