its cuz of you battery inside the comp it saves all the changes when you shut it down its prolly just dead open up your case and look for a round thin battery and just replace it.
It is probably your battery that has died find the battery on your motherboard(it usually is a silver thin round disc looking thing) and get the model number off of it and buy a replacement for it and then put the new one in.
I'm sure you might have heard this before but... Open up the Pc and inside on the MB you should find a silver battery (Looks much like a quarter) pop it out (Carefully) being sure not to break the tabs. Go to your local Radio Shack, toss it on the counter and say in a strong firm voice, "Give me another one just like it."
Go home, pop the new one back in place, set the time and you should be good to go for another 4 years or so.