my com is slow as hell
 

i dont know what happened i have been downlaodin stuff and i play halo (a computer game) and it is always fast but now when 4 people join my game it gets a ll slow and junk and when i minimize it my com is all slow and junk. i think i have a virus or something but i used adaware spybot and norton and found nothing also sometimes its slow even when halo isnt running plzz help