Battery leakage is the major cause of death old system boards. CPU and power supply fans can get clogged with dust and stop working. Old hard drives can develop bad sectors. Check this while your drive is still in manufacturer warranty. You should also have them professionally checked for viruses periodically.
Many people get viruses even with virus checkers installed.
Tips on Pleasant Computing 1. Turn off your computer when you are through for the day or going out. Unplug your modem line when not using the modem. If you live in a lightning area, unplug everything when not in use. Locate your computer at least 50 feet from the nearest tree.