Fan making noise!

Hello, I have a  one year and 8 month old Dell Inspiron 531 computer. The fan is making noise alot lately. It'll speed up and be loud for a few seconds, and it'll happen on and off. Other times the noise will be constant. A couple times it would make the noise, and all a sudden the computer shut off by itself. A couple other times when it made the noise my screen froze and it looked all pixelated. It seems to make the noise when I have more than three programs/windows open at once, but then again, it makes the noise when I only have one thing open at a time.

I contacted Microsoft about this, and they tried to fix the problem, and the noise actually stopped for awhile, but now it's doing it again. The guy that helped me over the chat told me to uninstall an antivirus program that I had on my computer, cause he said that was the problem; he also changed some other power setting too.But apparently those things didn't work!

Is the fan or harddrive going out on my computer? what else could be the problem?