The Ultima series (1981 — 1994) is the most important series in the development of computer role playing games. And it's getting easier to play them again.
Ultima I-III defined CRPGs, using tile based graphics and intricate worlds to immerse the player. Ultima IV was the huge leap forward, with a deep back story and a morality system that required you play carefully, not just kill and rob everything. Ultimas V and VI extended this formula into richly entertaining games. Ultima VII sets a landmark for non-linear gameplay. Ultima Underworld was one of the first 3d games, as pioneering as Wolfenstein 3D. Alas, Ultima IX broke my heart.
If you're too young to know these games, there's a lot of info online. But reading is dull: buy the Ultima Collection and play to your heart's content.
Except, well, PC technology has changed a lot and running the old games is nearly impossible. Enter preservation efforts. The most impressive is the Ultima Classics collection. "Sedryn Tyros" has collected the Ultima games and distributed them in a bundle along with DOSBox setups that make it easy to run the games. His supplement also includes original pre-PC versions of the early games, often better than the PC ports, along with the emulators you need to play them on a PC. Update: there's now a 1.3 release that combines the two. Alas, this collection is completely unauthorized and you'll have to scour your back alley's bittorrent site to find it.
Another option is fan-made reconstructions of the game engines. The best is Exult for Ultima VII, a portable engine that runs the classic game on many platforms (including Xbox!). Just take the open source game engine, copy over the assets from your Ultima Collection CD, and you're in business.
A lot of the first 20 years of movies are lost forever because no one preserved them. I fear the same thing is going to happen with games.