Your CMOS battery need to be changed.
It's a button type battery about the size of a nickel. There is only one thing looking like it.
Remove the battery and disconnect the charger.
Open the case, find the battery and remove it. You only need to pull back a small clip and the battery will pop up.
Bring it to any computer shop and ask for a replacement.
Put the new battery back in, FLAT side UP, and close the case.
Connect the charger and put the main batteryback in.
Start normaly. The BIOS settings could come up. If so, adjust the date and time.
Let it boot normaly. In Windows, check your date and time.
Electro, July 2012
