Why does my computer fan rev super fast? Do I have a virus?

Virus is an outside possibility, but the first thing I would think is heat. Are you using a laptop? I find that when my laptop is on non-conductive surfaces (like my bed), the fan will come on much more and much louder. Also, the fan will come on more as your computer is working harder (this heats it up).

If you have other reasons to think you have a virus, then I'd look into that, but the fan alone is not an incriminating symptom.

room temperature can also contribute to longer and louder fan use.

I agree with ben try putting it on a hard surface or in a cooler place and compare to see if it runs less. You can also buy a heat sink for the bottom of your laptop that cools it if you find it runs to hot.

but your thought could also be true -- my friend's computer (a Dell) was doing the same thing and then suddenly died, and it was from a virus --- there's no harm in backing everything up, whatever the cause