Severe performance drop after a few months.

Alright, here's my deal:

I got an Alienware M14xR2 last March (2013). It did a wonderful job for the first few months. I could run Battlefield 3 on near maximum settings, World of Tanks on maximum setting flawlessly, and countless other games without issue. There was no noticeable lag or graphical errors. 

This last August I left for college. Once settled in, I tried out some of these games out. Most were fine, but any online games suffered severe lag. I wrote this off as my dorm's wifi being garbage. I turned down the graphics settings in world of tanks and have had little issue. However, once back home I tried to set the graphics settings back to where they were before I left. Unfortunately, the games suffered the same issues as at school. This made it clear that it was not a matter of the wifi, but the computer. 

As another example, I present this: I downloaded Company of Heroes 2 from steam yesterday. I went in expecting to have a fine gameplay experience. I started the game with full graphics settings and immediately encountered terrible video lag. I turned them down to medium, and yet again encountered the lag. Finally I set them to the lowest settings and am still encountering severe lag. 

It seems that there is a general lack of information on how to go about fixing this, so that's why I'm here. Any help would be greatly appreciated.