Bad CMOS battery
Problem : I have a desktop computer and when I shut it down it resets the time/date to default time. It also resets the BIOS to default. Sometimes I get unusual error messages. Also some times if I view the information about the hard drive and/or the memory it is incorrect.
Cause : The most likely cause is the CMOS battery. You may be experiencing some or all the problems listed above. The CMOS battery is usually a small ( non-rechargeable) 3 volt Lithium coin cell. It is located on your motherboard. You should remove the battery from the motherboard, check the voltage on the battery, and get the number off the battery before buying a replacement battery.
Fix : Replace CMOS battery