Why are all my games so laggy (even on singleplayer) when they used to work fine?

I used to play all kinds of games on my laptop, battlefield 2142, counter strike and killing floor for example, they worked great, barely lagg, very smooth, but recently I've re-installed them and they're super slow, jittering every second or so. it's not an internet issue, I know that. could anyone shed some light? I've made lots of room on my computer, makes no difference