hmm... first I was wondering if you've overclocked anything or changed settings in the BIOS? Also, I know you've said you've tried everything you can think of, but did you do a system restore? If it only stopped recognizing it after a certain date, maybe you can save what you've done since then, and restore it back to that original date. It just seems weird that it was working, and now it is not out of nowhere.
Might wonder about viruses too. You can get a free online scan here: http://housecall.trendmicro.com/
It just does the scan online, so it is always updated. Might have to download something small though.