Low fps on steam

I have low fps on steam.In the nonsteam i have 100 fps all the the time.But in the steam i have 60 70 fps and it lags.How to increase the fps and smaller lag?I was watching people whit not very powerfull coputers but 100 fps all the time in steam.

http://img9.imageshack.us/img9/6445/85139087.png

Asrock 775i48
Nvidia Geforce 6200

Tell me how to get more fps and smaller lag?