slowmo on 04 January 2013
http://www.tomshardware.co.uk/forum/306303-12-windows-64bit-shows-73gb-usable-memory-installed
That's an example of someone with similar issues to you, they resolved the issue by reseating the memory a few times. I would recommend you do the same if this has worked previously as its unlikely to be a issue with the OS unless you have been fiddling prior to this occurring.
I'd also recommend while you have it open you inspect to see why your fans are running faster. It maybe worthwhile investing in a air duster and cleaning out the fans and heatsinks and ensuring there is no obstructions to the fans cooling the PC.
