My list of favorite "old" games would have to be something likes this...

Starcraft (RTS)
Homeworld 1/Cataclysm/2 (RTS)
Majesty (RTS with a twist)
Rome: Total War (RTS/Civ game)
Total Annihilation (RTS)

Heroes of Might & Magic III / IV (Turnbased Strategy)

Diablo II (Hack'n'Slash)

The Neverhood (Adventure)

Serious Sam (FPS)
Unreal Tournament (FPS) - And not the so-so 2003 etc versions. The good old UT
Deus Ex (FPS/RPG)

Neverwinter Nights (RPG)
The Elder Scrolls III - Morrowind (RPG)
Fallout 1/2 (RPG)

Think that just about covers it.... Lots of gems in that list - and by far most of them should run on even 5+ years old computers. Hope you enjoy at least some of them