leaving PC on = runs slower?

First of all, I'm not necessarilly looking for a solution to this problem, as there probably isn't one (although is there is, i would like to know it), I would just like to know why this happens.

Okay. The title pretty much explains it, but here's the story: I leave my PC on pretty much 24/7, except when it starts making very loud noise (another problem). The thing is, I've noticed that after a few days of being on, it runs slower, especially on games (expecially half-life 2), but also slower in general, sometimes VERY slow, as in, it-takes-about-10-sec-to-launch-Notepad. slow. Another thing I've noticed is that restarting it solves the problem, until it comes back. What's the reason for this?

 

Looks like you have all the symptoms of overheating going on there, the slowing down and the loud noise on startup. Check your CPU fan (and any other fans on the system)