Made a list of classic games that should be suitable even for very low end pc.

Fps:

Half life 1/2

Unreal/Unreal tournament

SiN Gold

Serious sam the first/second encounter/2

System shock 2

Deus ex gold edition

Arma cold war assault

Doom 3

Bioshock

F.E.A.R

Stalker shadow of chernobyl/Call of pripyat

Crysis 1

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Action:

Red faction guerilla

Splinter cell chaos theory

Metal Gear solid 2

Thief deadly shadows

Strategy:

Commandos (behind enemy lines/beyond the call of duty/2/3)

Age of empires 2

Heroes of might and magic 3

Starcraft

Stronghold HD

Command and conquer( Generals, Renegade, Tiberium Sun/Dawn, Red Alert series)

Civilization 3/4

Rome total war

Galactic civilizations 2

Supreme commander

Company of heroes

Racing:

Need for speed (underground 2, most wanted, carbon).

Flatout 2

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Rpg:

Diablo 2

Nox

Fallout 1/2/3/new vegas

Star wars knights of the old republic / Jedi Outcast Jedi Academy

The elder scrolls daggerfall/morrowind/oblivion

The witcher

Mass effect 1/2/3

Dragon age Origins

Puzzle:

Portal

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Feel free to suggest more games for the list.