I meet all the system requirements for tf2 but it still lags

I couldn't find anything about this anywhere...

so I've decided to actually ask for help myself. like the topic says i have all the system requirements for Team fortress but it lags still, i have no idea whats wrong, I have tried lowering the graphics but that doesn't work... i'm confused

please help me

System Specs:
Processor - AMD Phenom(tm) 9500 Quad-Core Processor
Video Card - NVIDIA GeForce GTX 660
Ram - 4.1 GB
Operating System - Microsoft Windows 7 Home Premium Edition Service Pack 1 (build 7601), 64-bit

I meet all the system requirements for tf2 but it still lags