Windows Vista time settings on laptop

anyone help with this one,Sony vaio laptop time setting keep changing,when i set the time and save it,then power of ,on starting up again the time has changed again.i set time lots of times but still keeps changing.

ta paddy:loco:
Best bet is the CMOS battery is gone as said above, replaced mine and my time stays put now. :). How long have you had the laptop for?