Boot with a Dos floppy or a win 98 startup disk. The OS doesn't matter.
Happy flashing
Ethan
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. "Why shouldn't the American people take half my money from me? I took it all from them." - Edward Filene
Follow any and all instructions for flashing your Bios and make sure you understand them. The Bios procedure is done outside of Windows so XP is not used at all.
Actually, I remember seeing in XP Pro, in the dialogue where you format a floppy disk, an option to make an MSDOS boot disk.
You don't really want to use a Windows 98 startup disk (unless you rename the autoexec.bat and config.sys files on the floppy afterwards). What you want is a clean DOS boot, with no extra drivers or memory managers loaded for bios flashing.
Alternatively get to a win9x machine and type sys a: or format a: /s to make a bare minimum DOS bootdisk.
Creates a plain disk, with a blank config.sys file. The only other stuff on the disk are keyboard maps and stuff, which unless loaded in the configuration files are just files on disk.
That disk would be suitable for bios flashing purposes.