Clock on Computer goes back an hour

I have been having trouble with the clock in the lower right corner of the computer screen just going back an hour every once in awhile. It doesn't do it everyday. But it has been going on since the clocks changed. We are on EDST and it goes back one hour. I have run Norton Antiviues and there is no indication there is a virius. I have run the LiveUpdate on both the Norton Systems 2003 twice a week and well has the Firewall 2004. I have run the Spybot and still nothing. But the computer has been running slow at times. I downloaded a MS Maginifer from there website and the computer ran even slower. So a few days I unintalled it. It was doing what I wanted to do anyway.  DSoes anyone that any sugguestion?

Thanks,