Etherjet, pxe and why does it boot from lan?

Hello,

I have here a relatively old ibm desktop computer (300pl), which is a
pentium 3 machine. It has an ethernet card in it: ibm etherjet "with
alert on lan". If I try to use that card, the computer refuses to boot
from anything other than the LAN (with PXE). If I remove the card or
swap it with a 3com, there's no problem and I can boot from floppy.

What intruiges me: why doesn't this card seem to have a removable pxe
chip? If this was the card the machine came with, why was it fitted
with a hard drive? Would it be possible to change the boot order in
the bios? (I can't find that option).