My PC system clock slows down approx. 5 minutes a week. What do I do? Do I change my MB's CMOS battery?

Try that first. How old is the computer?

My computer is 5 years old. Maybe it's a remedy. Thanks.

Five years is a reasonable life expectancy particularly if the computer isn't used very much. Before changing the battery, look up the motherboard's manual and see if there are any things to be considered during battery replacement.