You are taking a lot of risk running a beta operating system.
In order to safely flash your bios you should be running in the equivalent of plain old dos with no memory management running. If you know how to boot in such a mode in WindowsXP you could flash from the hard drive. In Windows98 this is called safe-mode command-line.
I wouldn't try creating a bios flashing floppy with your beta WindowsXP system because it may have the Windows2000 floppy corruption problem until you are able to upgrade the bios.
The way I would do it is to format a floppy using a Windows98 machine with the system files checkbox checked. Then copy the bios flasher and bios rom onto that floppy using a Windows98 machine.
Then make sure your WindowsXP system will boot from a floppy first, then boot from the Windows98 floppy. Hopefully at this point you are running in Windows98 command line safe mode which is where you want to be. If you are then run the proper command line command to flash the bios. Include the .rom extension when you type in the command. I believe it would be
nameofflashprogram nameofromtoflash.rom
Once the bios is flashed and the computer restarts reset the bios settings to a default setting, then reintitute any customizations you had made.(example, like turning off on-board sound)