Why is my computer fan making noise?

It's overheating. Do you have a pet? Cat or dog hair inside the box is often the culprit.

I have a cat and dog. I looked inside the desktop; there was some dust but cleaned it out.

If it's been cleaned out (and the rest of the interior of the computer of all dust and) it still makes noise, the fan needs replacing. The bearings in the fan wear out.

Well, that has nothing to do with fan noise, does it?

{edit: Why did you just delete your reply about having a virus message?}

First you need to determine whether it's a false message or not. Sometimes the message is actually a spyware program...trying to put pop-up ads on your computer. Run your virus checking program.
If your computer has a virus, then you need to run a virus repair program.