Blue screen: unable to write to disk c:

I’ve been getting this blue screen for a couple of days. I can hit enter to return to Windows and after 3 or 4 enters it will stay until the next complex redraw. Often I work for quite awhile in CorelDraw or Excel without a problem. I’m unable to do a through scan — when trying to do so with an emergency disk I got an “unable to find a valid FAT” error. The problem has occurred a few times.
I recently installed “SystemWorks” and I used “GoBack”,which came on the system (a SystemMax Athlon 650) to revert the drive a couple of days. With no success.