Cannot open BIOS for changing the settings

If you want to install operating system through bootable operating system CD/DVD and you are not able to make any changes in BIOS like changing the first boot sequence to CD/DVD drive, then you have to first clear or reset the BIOS settings to factory default and make any changes you desire.

Procedure:

Switch of the computer. Open the CPU cabinet and look for the battery placed on the motherboard. 

 Remove the battery and switch on the computer. Now you should be able to see " CMOS ERROR " press F2 or F10 or DEL to enter BIOS. The error message varies depending on the manufacturers of the motherboard and BIOS. Switch off the computer now and put the battery back to motherboard and switch on the computer and press F2 or F10 or DEL to enter BIOS depending on your BIOS manufacturers. You should now be able to make any changes to BIOS. Remember, any changes made to the BIOS will not be saved if there is no battery placed on the motherboard.