FPS lagging problem all of a sudden? (games ran smooth but suddenly lag)

Well I have been playing it for about a week now and it runs smooth maxed out and 4x AA. Now all of a sudden it runs at like 3 FPs and even when I switch to low settings its 10 FPS. WTF is going on? I've tried rebooting and ending most of the big processes in task manager. Also it runs fine in single player as normal but multiplayer is totally messed up n laggy. I have current date drivers and Game Boost which helped to increase FPS a bit so could it be that? because it started the second time I used BFBC2 with gameboost on. I run it through Steam too. Please help this is annoying. Oh and if this is important when I start playing CPU usage jumps to 100%. I'm not sure if it did that before.
 
wwhat are your system specs? and please try updating all your drivers before trying anything else
 
Back