|No but you CMOS battery probably is.|
The CMOS battery holds the hardware settings when you turn the computer off. The fact that you can go into the BIOS and select the hard disk and load Windows is a good indication of this. Check the time as well. If the time is wrong that will confirm that the battery needs replacing.
How you do this is dependant on whether you have a desktop of a laptop. With a desktop you will find the battery on the motherboard. It is a small silver coin battery of the type you will find in cameras. Will all power removed, replace it.
After replacing, boot back to the BIOS, check the settings and the time and any other settings that may need to be adjusted and save them. Usually by pressing F10
Reboot and you should be fine.