My slow Mac

My Mac desktop computer is about 4 years old. Over the past few months it has began to run slow, and when I try to download anything it reads "mounting failed," in the little download box. Now I was told to take all my pictures and important personal stuff off of the computer, and restore the computer back to its factory settings. Now I am willing to try everything, but do you think restoring it back to its factory settings would help? If it would can you please explain to me how to do that? I have tried everything besides taking it in to the apple store and paying a bunch of money to have it looked at, which I'm trying to avoid. I rather fix it at home if I can. please give me your opinion. Thanks..