i have joined two pcs together at home and the main one is attached to a
576kbps braodband connection, the other pc is joined via two ethernet cards
and a rj45 cable. now everthing works fine but what i would like to do is set
how much of the braodband connection the other pc can have. lets say 25% for
an example. now how do i do it?

Windows connection sharing has very limited features and what you are asking
for is not possible. You will need to install a third party transparent proxy
or routing software.  Most of which you will have to pay for, a quick google
search will give you plenty of options.
However most of your search results will turn into you been recommend that
you either install a server edition of windows or a linux distro (IP tables).
There are a few and if you have trouble finding some then please post box.

i have tried a numerous amount of software to get this to do what iwant but
most of it is too difficult for me to work out or the ones i could get to do
what i wanted caused other prorams like msn and even my pc performance was
reduced (dodgy software)

could do with a step by step i dotn think that many people have looked into
this tbh i only have 576kbps braodband from wanadoo/orange and they say its
not available any higher in my area yet that would solve my problems anyway
to have at least 1 meg.

Can't ping a Windows XP machine
home network problem
help SP2 and connecting
Problem: No Network Connections using Guest Account
Briefcase is slow
Can't connect using RDP from one spot but can from another
Cannot access local network - pings successfully + can access by I
Accessing My documents
Delay while pinging domain name
"network name no longer available" error during large cifs copy