Sorry, I may have misunderstood your question.
If you meant how to download the new BIOS, then:
- get into Windows
- goto
http://support.asus.com/download/download.aspx?SLanguage=en-us
- select product Motherboard, select series Socket775, select models P5GDC Deluxe
- click on the BIOS tab
- pick the BIOS you want and click on Global, USA, Europe or China to start the download
- the dowload is a zip file - unzip it to a Windows file where you can find it later
- click on the Manual tab
- download the manual in the language you want
- in chapter two it describes different ways to load the BIOS into the PC, including making a bootable floppy disk to do the BIOS update in DOS mode using the AFUDOS program from Asus
If you meant how to save the current BIOS before loading the new one, then:
- make a bootable floppy for DOS as they say in the manual
- add the AFUDOS.EXE file to the floppy as they say in section 2.1.3
- don't put the new BIOS on this floppy
- use the floppy to boot into DOS
- use AFUDOS /oSAVEBIOS.ROM command to save the current BIOS
- then load the new BIOS as they describe in the manual
Hope this and the manual answer your question.