Your video card won't support the knoppix default video mode, so starting with knoppix vga=normal should work OK.

(WARNING: I don't know exactly how knoppix boots from floppy, if it's a lilo floppy or ???)
If it uses lilo and you want a permanent fix so you will boot automatically in the correct video mode you will have to edit your lilo.conf file (as root) and change the video mode in there. FIRST make a backup of your existing lilo.conf file just in case!
( cp /etc/lilo.conf /etc/lilo.conf.bak )

Your /etc/lilo.conf file should have a section near the top like this:
<snip>
#
lba32
#
vga=791
#
</snip>

put a hash ( # ) in front of the vga=791 so it looks like this:

#
#vga=791
#

save the file, and then make sure to rerun lilo, with the command:

/sbin/lilo -v

(if you don't rerun lilo the changes won't take)

I can't guarantee this will work with a boot floppy.

HTH

rock