Thief 3 and Deus Ex are nothing to brag about. They aren't exactly demanding games by todays standard of computers.
Forcing your computer to do something it wasn't made to do can be risky. Games and netbooks are not best buds, and if you aren't careful, you could literally melt your hardware and casing. You could even void its warranty.
Im also going to take a stab and assume you are running an Intel graphics chip, which is below the requirements for the game.