It could be the hard drive has become faulty. All hard drives fail sooner or later. Test it with the drive manufacturer's diagnostic software for DOS
which loads from a bootable CD (you cannot test it properly with Windows software).
The links to download the software are at the top of this page: http://www.tacktech.com/display.cfm?ttid=287
Test the RAM modules too, with Memtest86+
To make a bootable CD from any ISO file, you can use IMGBurn