Concerning the old system:
Although I'm temped to go balls-out and list some of the best games out there, I will only indulge myself until I get tired and can not think of a single other game that will run on your system...
Little Big Adventure (a.k.a. RELENTLESS: Twinsen's Adventure)
Little Big Adventure 2 (a.k.a. Twinsen's Odyssey)
Oddworld: Abe's Oddyssey
Oddworld: Abe's Exoddus
Heart of Darkness
Out of This World
Flashback
Prince of Persia 2 (KICKS ASS)
Day of the Tentacle
Loom
Monkey Island 1-4 (YES I SAID FOUR)
Sam & Max HIT THE ROAD
The Dig
Diablo
Freddy Pharkas Frontier Pharmacist
The Adventures of Lomax
Space Quest 1-6
Leisure Suit Larry 1-7 (please note that LSL4 does not exist)
The Adventures of Willy Beamuth
Alone in the Dark
Broken Sword 1 and 2
Worms (!!!)
Lemmings (!!!)
Beneath a Steel Sky
Stunts (this is more of a joke)
Civilization
StarCraft
Dune II
Command & Conquer
WarCraft II
Legend of Kyrandia 1 - 3
Discworld (go for 2, unless you want to be frustrated, or a hardcore adventure fan)
Discoworld Noir (kicks so much ass)
Grim Fandango (if you have not played this you have not lived)
SYSTEM SHOCK 2
DEUS EX
I have tons of others, but I'm informed by a friend that I have to post in some other, less entertaining topic...
There are literally TONS of great old PC games. In fact, so much, that you should look here:
http://www.the-underdogs.info/
Happy Playing.
EDIT: The Bold games that I thought you could not live without.