Programs Lag When Minimizing, Warcraft Game Lags?

I've been using this windows 7 desktop for over a year, and everything has been fine until the past few days. When I minimize a program or when i maximize it when it is minimized, the program lags. For example, when I am running firefox and when i click the minimize button, instead of firefox minimizing in a swift motion, it minimizes slowly in other words, lagging. Also, when I play Warcraft the game lags. It lags in a really strange way, almost as if the game is drifting around when I move my mouse. Usually when games lag, the controls you make respond slowly, or it is in a jerky kind of way. I've tried restarting my computer many times, and all is well for the first 15 mins or so, before this lag kicks in.