Were you able to create a floppy from the file? there are 2 files in the dell support web page for your dimension 8400 under SATA drivers you will need one of them (downloaded file that has been run to create the floppy) when you are hitting the "f6" key loading windows.
yes, the file extracts to a detected floppy disk.
The problem comes after F6, I hit "S" to specify drivers, and shortly after the process starts I revieve the "unexpected response" error.
when you hit "s" to specify drivers did you try all of them on the disk? have you downloaded the other file and made a second floppy to try?