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Return to Castle Wolfenstein FTW!

The Elder Scrolls III: Morrowind

Terraria, Total War Rome, Bioshock 1 (maybe), Prison Architect, FarSky, GTA Vice City/San Andreas, Portal 1, Age of Empires III, Heroes of the Might and Magic V, Torchlight, FTL: Faster than Light

a few amazing games I played on a VERY low end laptop :)

Tetris.

Depends what he likes.

Counter Strike: Source

Painkiller: Black Edition, Might & Magic: Clash of Heroes, any King's Bounty game (though I'd personally recommend Armored Princess/Crossworlds), should also be able to run all the Serious Sam games except 3, pretty much any Worms game (can't say anything about the newest one but they are easy on the requirements in general)... I dunno, there are a LOT of good low performance games. What genres is your friend interested in?

Brutal Doom

Warcraft III: Reign of Chaos & Frozen Throne Expansion

Super Meat Boy

VVVVVV

Call of Cthulhu: Dark Corners of the Earth

Fallout 1 and 2

Quake 1 and 2 (texture mods available)

Serious Sam: the first encounter and the second encounter

FTL: Faster than light

Sword of the stars: the pit

Hitman: Bloodmoney

Hitman: Contracts

Jet set radio

Majesty 2

Oddworld: Abe's Oddysee

Oddworld: Abe's Exoddus

Age of empires 2 hd remake

Dawn of war 1 + expansions

Zombie Driver HD

Teleglitch

Luftrausers

Evil Dead: regeneration

Half life 2

Black Mesa (check requirements) free mod

Killing Floor

Doom 3 + expansion

System Shock

Deus Ex 1 (considered one of the best pc games of all time)

Condemned - Criminal Origins

Obscure 1 and 2

Marvel Ultimate Alliance

Silent Hill 2

Tomb raider Legend

Tomb raider anniversary

Tomb raider underworld

X-Men Legends II Rise Of Apocalypse

Sim City 4 + rush hour expansion

Rayman

...and a shit load more

Age of empires!

Age of Mythology

I have a pc with similar(or lower) specs, so here's the games I play in it:
Dynamite Jack
Uplink(haven't played for a while but it's really fun for a certain kind of people)
Cave Story+(awesomeness)
FTL(somehow the resolution of the game is too big for my pc :/)
Super Hexagon(it has a Super, so it must be Super furtra...er I mean fun)
Organ Trail.
CS 1.6
DoD
Fallout.

GTA III/VS/SA
Hitman 1-4
Half-Life 1/2
Mafia: City of the Lost Heaven
Dragon Age Origins (maybe)

Terraria

Zork is guaranteed on pretty much anything ;)

Maybe some point and click adventure game? : D

It's so insensitive and rude of you to call it slow. Performance challenged is the form we'd like you to stick to.

mario bros?

StarCraft.

lots of awesome retro games like VVVVVV or Love+

The elder scrolls: Arena

Puzzle games

Don't starve