Hello Albert and
to the forums
You can tweak your pc to get the best out of your pc but officially there is no legal tweak for the modem. If you are having speed issues do a speed test and post them here. the members who work for NTL will look and ask you to contact them if their is a problem that they need to look at.
I am on 750K
This is one I did just now I get good speeds late at night but can be slow during the day.
ADSL speed test