Your hard drive could be failing (as mine just did). From what I've been reading already, IBM, Hitachi, and WD do not get the Internet tech community's general consensus. Maxtor, Seagate, and Samsung supposedly do.
In any case, you should download a tool to analyze the SMART information of your drive (something like this
http://www.passmark.com/products/diskcheckup.htm). While you look up reliability information specific to your drive, start backing up. Good luck!