If you like the old school games, or enjoy just going back to the good ol days, here's your chance.
Doom 95 is the way to go, there is nothing else needed to download, it comes with the Windows program to run it, and comes with shareware version of Doom I. If you have or can find other's it will work with them too.
For more things you can go to
DoomWorld's shareware section (Provided
here). It has everything you could want from Doom I shareware to Heretic and Hexen shareware, and even programs to download to make your own.
If you are more advanced or just want to take a more advanced approach at it, you can go to
ZDoom and download their most recent version of ZDoom, which is a little more complex at getting to work (involves knowing the DOS version commands when setting up) than the Windows version. You can download the most recent version of ZDoom (2.3.1)
here. Just click whether you use Linux or Windows. You will have to provide your own IWad (Version of the game) though.
(Windows versions supported by ZDoom :: 95 / 98 / ME / NT / 2000 / XP)