You dont have to buy new ones, you paid so you can play thoose games so (in my view) it would be ok for you to make a copy of the disc from a friend who has it, as long as you have a valid serial key that is.
Doom 3 has no copy protection, so you will not have any problems with that one. as for 24, i guess it's based on the tv series.... btu i have never heard of it, so i dunno :P
PS avoid the P2P networks, even though you own a license you can still get into trouble.... due to the nature of P2P you could be helping somebody who doesnt own a serial key get a copy of the game, which counts as illegal.