Bad Performance for No Reason

... on games I've played in the past.

Hell's Highway and Borderlands 2 are virtually unplayable. I don't understand. I know I have a dated system, but I remember having played these games in the past, and they ran smoothly.
And it's funny because BL2, with the .ini tweaks, runs at about 60fps on my friend's system, which is identical to my own, except that it has a weaker GPU.

Since August, games have been running annoyingly slowly. The game starts off at about 30fps, and eventually drops down to unplayable choppiness.

You should check the temperature, something may be running too hot. Happens to me if I've been doing something resource intensive all day then try to game. And I've been too busy to actually clean my pc.