Open it up and blow it out with some air (compressed or low psi from an oiless). Leave it open and run it and see if your having the same issues. I assume the computer isn't turning off randomly or your would have mentioned it so i doubt its over heating too bad because you would end up killing your hardware, signs of that would be random shut downs or just acting funky in general. You could have a fan going out but in all honesty that old of a computer i would just get something newer if you can afford it or just hit me up and we can see about you building one yourself (very easy to do and I can help if needed).