Ok, I posted about this a few days ago... Here's what you need to do... First, how old is your PC and is it still under warranty? You need to get a replacement chipset fan. This fan is the silver fan below your ram sticks. This is a known problem with our motherboards that Asus has since addressed with a newer model chipset fan. If you have a warranty, contact your PC manufacturer and get them to send you a new fan. Now, the hard part is replacing it. The fan is held in by two barbed, spring loaded plastic pins. To remove these pins, you either have to remove your whole motherboard, or you have to use a pair of super strong wire cutters to snip the clip off as close to your mobo as possible. If you are lucky, the stub of a pin that is left over will fall out of its hole allowing you to place the new fan on the board. I got lucky and succeeded with the wire cutters. Once you remove the old fan you have to clean the chip off with some rubbing alcohol and Q-tips. This takes a while to remove all the old thermal residue, but it is still good to do to ensure a nice contact between the new thermal tape and the chip. Asus was really jerky about this fan, and should have included a better solution, but such is the way of things
Good luck!