BTW, Fish40, those online games are notorious for embedding ad-ware onto a machine. A friend of mine was having the same exact PC problem a couple of years ago and asked me to take a look at it for him. Using a free Adware scanner, I found over 400 ad-ware programs on his PC, all eating up precious resources. I also found a nasty little program, disguised to look like an adware scanner, that was exceptionally hard to remove. Long story short. The Adware scanner did the trick and he was up and running again in no time. I'm not saying that this is the same cause in your case but, it might be worth a shot, and, save you some dough.
I'd go easy on the dust filter though. It will reduce airflow through the system, causing higher temperatures.