need to reformatt and downgrade/older system

I asked before only how to reformat my
computer,unfortunately,I failed to mention that I have an
older computer and it cannot tolerate the xp,it's slower
than a snail and takes up alot of space.Atleast i think so
anyways.I need to get rid of everything on my computer and
start from scratch with win98.I need to start the puter in
MS-DOS,but don't know how.I still have both the disc and
cd but winXP wont let me do anything.Someone PLEASE help.I
did get the response from someone but it came with an addy
for the manufacturer.Does that apply to 98 too?Please have
patience with me I'm fairly new at this computer stuff,I'm
self taught.That doesn't say much.(haha)
Thanks ahead of time ,
Shari