When i was on dial up i noticed on occasion that things had slowed quite a bit. I had Norton installed but the reason things slowed down was because Norton was updating, when this process had finished there was no problem.
I know when one first installs Norton, the updates can be huge so perhaps this may be the cause.....It aint finished updating?
This is worth a go (is for Norton antivirus (NAV) 2004 but may work for an earlier version too) - click here.
If it's Norton internet security(NIS)/personal firewall try this - click here. Good luck, G
PS - I have NIS2003 (with NAV incl), with UKOnline dialup (on Win98se) but don't have a slow down. Some freeware AV programs to consider if you cannot solve your problem - click here (preferably uninstall/disable NAV before installing).