Most old DOS games can be run on low spec computer. You just need a DOS emulator (DOSBox) to run them on Windows 7.
But if you are looking for 3D, high graphics games
then you probably don't want to play DOS games.
Many old DOS games are abandonware and can be "legally" downloaded from various abandonware sites. I have hundreds of DOS games (Jazz Jackrabbit, Doom 1, Doom 2, Carmageddon, Commander Keen series etc...) I play on my Windows 7 laptop.