Run this on the computer:http://www.download.com/Trend-Micro-...-10227353.html
Then save the file(Should be a notepad fire or .txt file) and go to this site and copy and paste the whole file into the analyze box and click analyze. If anything should come up with a red 'X' go back to your computer, run the program again, and find the EXACT file name in the program that has the red 'X' on the website and check the box next to it. After you check all the boxes with red 'X's' in them, go to the bottom left and click fix.
That program should find the running process that is taking up all the CPU usage and by checking the items and clicking fix, you delete it. You can also delete yellow 'X's' while your at it.
I had the same 100% CPU problem and it was caused by a virus that was acting as a windows virus in my temp folder. AVG wouldn't pick it up but hijackThis did and saved me a reformat.
The fan is sounding loud because your CPU is producing alot of heat from being at 100% all the time, if your wondering, deleteing the process that is revving it up to 100% is deleted, it should stop.