iMac Fan runs fast and hard
Not normal. I had the same problem with an iMac a few years back and had to have the logic board replaced (under warranty thankfully).

You might have a SMC (System Management Controller) problem. This article explains how to reset it: 
http://support.apple.com/kb/HT3964?viewlocale=en_US&amp;locale=en_US

You might also look at a tool like iStat Menus:
http://bjango.com/mac/istatmenus/

What I found with iStat was that my CPU was running extra hot, hence the extra fan speed.

I did end up using an app called SMC Fan Control to mitigate the problem before getting the logic board replaced.