Why is Counter Strike Source lagging so much?

when i play online or with bots its so laggy i don know why or its my INTERNET ? but i got nice COMP (sony RB) and it has 2.8 Ghz and 1 gb ram it got INTEL (R) 82945G Express Chipset Family video card maybe its my VIDEO CARD is lagging but in other games it doesnt lag like COD,NFS,C&C i play well i dont know what is probem with CSSS?????