Computer system clock has to be reset each time computer is started. It is sometimes hours off the correct time.

This is probably because the battery on your motherboard is dead. You'll have to get a replacement either from some old PC you have lying around (the batteries of all motherboards are the same) or buy one in your local PC store or online.
Open up your PC case and when you look on your motherboard you will see a little round thing containing a battery. Simply click out the old battery and put in the new one. Then set your system time again and it shouldn't reset anymore for a very long time.