+1 to Ext64. I've not had any trouble with that. My fans will all spin up as the computer boots and loads everything, but they'll all drop to normal idle as soon as I get logged in and loaded. I would just do a real good clean (uninstall, reboot and run driversweeper in safemode, etc.) and then reinstall a clean copy and see how that works. If it doesn't do it, just manually set the fan speed to a reasonable level (~40% is generally pretty quiet and keeps things cool enough.)
The fan will run at full speed after you first boot the computer and through the windows startup. It is not until the video driver loads during the windows start up that the fan will slow down to the preset speed as determined by the CCC config and driver settings.
The fan *could* be running at speed for 10-15 minutes because of the CCC config and driver settings.
It would run high for booting, but 10-15min after that is pretty long. Just try setting the fan manually to something more acceptable. I guess that may be closer to 30% with a 5850, (with my 5750s its closer to 40%).