Likely you need replace the cmos battery. If it's dead, or nearly so, it won't hold the settings. Then when you boot up the bios detects that the settings have defaulted and asks if you want to go into cmos/bios setup and correct things. Otherwise, hitting F2 continues the bootup with the default settings. If it's not the battery you may have a bios problem. Sometimes you can correct it by selecting the 'reset configuration data' in cmos.
Other times you can clear the cmos. There should be a 3-pin jumper on the motherboard. It would now be jumpered in the 'normal' position. To clear it you'd move the jumper to the 'clear' position, turn the PC on for a few seconds, turn it off and then return the jumper to the 'normal' position. On next boot you'd be alerted that you need to enter setup.