Your date and time is set incorrectly, usually. Your system thinks it's 1994 again.
Replace the CMOS battery on the motherboard, it's a little disk shaped watch battery that you can buy at radio shack. Then set the date and time. Then everything should work OK again.
When the battery is old, it stops keeping track of time, and the security certificates don't match the dates in your system, that's why you get the error.