BIOS Flashing 101:
Method 1, using the BIOS' built in too:
Download the latest bios file from your manufacturers site. stick it onto a cd, floppy disk, flash drive or other removable media, make sure it is the only file on there. Insert said media into computer and reboot. enter the bios by pressing a key (in most cases it is DEL) and locating the flashing tool. Example from an Asus P5N-E Sli motherboard:
It should be straightfoward to use it.
Method 2, Flashing inside windows:
Note: This should always be done in safe mode. tapping F8 during boot will allow you to select how you would like to boot windows.
Download the latest BIOS file from your manufacturers website, as well as the WinFlash utility (some will come with an installer of winflash) if it is not included. Extract the BIOS file to a new folder, and then open WinFlash. Select File > Open. Browse to the bios file and open it as seen here:
Select which parts of the CMOS you would like to flash (it is recommended to select all EXCEPT the boot block. if the flash fails and wrote over the boot block, the motherboard will become useless as it cannot boot from anything!) as seen here:
And then select File > Update BIOS or press the icon, like so:
The program will then proceed to update Your BIOS:
And will ask you to reboot when finished, the changes won't take effect until you reboot: