Luckily, you can buy Doom 1 and 2 on Steam for 9.99€! Even better, there is a Doom pack for 34.99€ which includes: The Ultimate Doom, Final Doom, Doom 2, Master Levels for Doom 2, Doom 3 and Doom 3 Ressurection of Evil.
Doom 1 (9.99€)
Doom 2 (9.99€)
Ofcourse, if you would like to play Doom 1 or 2 as it was supposed to play, then you must have DOS or at least FreeDOS which is a rear thing today as Loch Ness. Anyway, if you use Windows XP/7 you have to use DOSBox, DOS Windows emulator.
To run any old DOS game, including Doom in Windows XP or 7, download DOSbox here.