If this is 2000 or XP- he may be lying, a bog standard retail OEM key works only with OEM keys and a Dell, HP or other manufacturers key only works (*) with the correct CD from that manufacturer.
- Vista, 2008 and 7 do not even need keys if you have the correct CDs
However, I have a collection of original CDs from Dell and other manufacturers that I see on a daily basis and I use the correct edition when I see the correct licence if/when I need to do a restore.
Is it illegal - I don't think so - IANAL, but I really do not see the problem on original media if you only install on legally licensed machines - at the end of the day, if you lose the media, you pay £xx and will just get the same cd in the post.
... As for burned media, just be careful - if you do not know where it came from, you could be walking in to a lot of other problems later on.
@Luminose, Arjan van Bentem has a very good point. is this possible? – studiohack – 2010-03-29T04:31:41.260
You shouldn't need the CD ever once Windows is installed unless you need to do a repair of the OS. If I remember correctly Windows copies all CAB files to the HDD during installtion, negating the need for the CD. – Juice – 2010-04-05T21:35:08.890
3Apart from this being legal or not: I think it can really get the owner into technical trouble if a somewhat different installation disc is used, even when the key is accepted. Some time later, when the cd is needed (like when installing some new hardware or whatever) Windows might reject the original disc because it does not exactly match the installed version, and prompt for the cd that was used during repair? – Arjan – 2009-12-26T08:47:12.683