The Definitive Shooter that defined the first person shooter category. the plot involves a wormhole experiment that goes wrong and opens a portal to hell on the moons of mars. This is the 4 episode retail edition witch includes an prequel episode to Doom 2. The game runs great on a net-book and is still a great play you can get the game on steam in a dosbox wrapper or extract the doom wad file and use a source port to run the game naively in windows. The port i recommend is the Zdoom port witch can run almost  all of the doom engine titles.

there are two places to get this title online from ID’s own online store and Steam. the major diffrence between the two versions is that the Steam edition is the DOS exe running in dosbox with out the old setup program so you are locked to WASD movement controls. The ID store download is the DOOM 95 Windows port by microsoft it requires the dplay.dll file added to the install directory to run in windows 7

home page http://www.idsoftware.com/games/doom/doom-ultimate/

Steam page http://store.steampowered.com/app/2280/

zdoom page http://zdoom.org/News

shareware download http://www.doomworld.com/idgames/index.php?id=7053