It may be a graphics card. Just remove it from the mother board, clean the edge connector with a soft eraser and put it back in.
The mother board has CMOS settings. These are the settings for time, date, memory, type of hard drive, floppy, etc. There is typically a battery that backs this up and it is also on the mother board.
The DOS and machine software is on the hard drive. You lode that if you have a hard drive crash or a nasty virus.