Thanks for the advice, but I'm almost 100% sure it's not a software problem. I've had this setup running since 2004 without any problems. I've also formatted a bunch of times since I put the computer together, and formatted once since this problem started.
Here's my power supply (Antec TRUE430 430W ATX): http://www.newegg.com/Product/Product.aspx?Item=N82E16817103908
Here's my power supply (Antec TRUE430 430W ATX): http://www.newegg.com/Product/Product.aspx?Item=N82E16817103908