Anyone remember X-COM: UFO Defense/ Enemy Unknown (USA/ European version), and X-COM: Terror from the Deep?  Weren't they great games?

Well you can get them for XP (well, it's worked for me on XP) on fileplanet (google it or something) now that they count as abandonware!  Hours of Old-School fun. 

Also, Wolfenstein 3-D is around somewhere.

Wolfenstein 3D? Dearie me, I must download! I still remember how silly that mecha Hitler was.   Of course back in those days I nearly wet myself with terror.