Old Bios not recognising New 80 gig hdd. (maxtor)

I have a rather old compaq deskpro, 647mhz intel processor running XP
(without any boot diskettes) , i have had a 10 gig hdd since i bought the
computer fer a tenner, a year ago, and i put a 4 gig slave drive in it. Now
i know these are small drives, but I have just bought an 80 gig maxtor hdd,
and when i insert it, in place of the slave, it is not recognised..altho, in
ctrl settings/system/ its THERE.. and when i ran my computer using it as the
master, (i was gonna install xp on the maxtor 80) it only found 16 of the 80
gig , something about the bios being old and ya have to run a maxtor app to
reclaim some more of the hdd.

Right...as i have no boot diskettes/disks , how do i get to even install
this hdd, as it does not give me the option to partition the disk, or
anything...BUT i also have 3 other hdd's all about 4-5 gig, and NONE of
them, when i used them as slave drives needed partitioning, or tell the
computer to make them a d: slave, (C: master) it just was! - no input needed
from me.

So if i take my master(10 gig) off the computer, there goes my o/s , i have
an installation disk (xp its worked on my 10gig fer months, not new, not
even orig.) so what shud be my 1st move, thats all i want to know, should i
keep with the making it a slave, and take the 16 if it is offered to me, and
hope that maxtor have a program/app that gives me some of the wasted memory,
?

Do I have to format the hdd(80)? how do i even create a bootable disk to
start the whole installation off? any help is good help at this point... my
bios is old , 1999, 686T2 , it seems shitty to have to take only 16gig out
of a brand new 80 gig hard disk?
what do you advise please?