Counter-Strike: Source lag

Hey guys..
me and some friends is started to play css agian, but, my friends computers are 3-4 years old and not so good gaming computer anymore, my is only 1 and a half year old and I got like 40 fps in a de_dust2 map? my friends with a less good computer than mine, has a fps over 100? I can play without lag if I minimize my full screen size and if I set all my quality to low, what do I do ? I tried to update my driver, and I reinstalled css but nothing helps, I just lag a bit, and specially when I go into a smoke grenade, it lags like hell ? please someone help me ! my computer is a
Acer Aspire 5742G

Intel Core i5-480M
Nvidia GeForce GT 540M
6GB DDR3 Memory
640 GB harddisk

If I go to side called canirunit.com it says my pc can play css 110 % , but I cant ? :S
 
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