Beeping noise then hectic lag

[maxtrouble]

Hey people for sometime now i have been running games fine on my rig sporting i5 750 and a rather hot gtx470. while i was playing prototype 2 today i received constant lag which was accompanied by a beeping sound. if i press shift once then it stops but its irritating while im running(w button) Any help?

[THE_STIG]

sounds like overheating

But then you mentioned that pressing a key has something to do with it, could be that you have sticky keys enabled \

[maxtrouble]

Thanks but ive checked and its not sticky keys