Windows 7 , 64 Bit , 4 GB Ram.
Processor : Celeron Dual Core CPU T3000 @ 1.80 GHz 1.80 GHz.
Also , i have the same problem with Battlefield 3.
Weird fact : MW3 is running fine , but Black Ops and Battlefield 3 are both laggy as hell ... While MW3 seems to take more RAM memory while it is running ... So yeah , i dont have a shitty PC , cus these are the first games i ever have problems with ... Hope u guys can help
EDIT : And its not my net , cus i have LAN , and i dont have any problems for any other games :P