Especially on older systems, Norton Internet Security has a _very_ high
overhead. On one system I work with, we have Norton Internet Security
disabled and NAV disabled except for the email checking. That has close to
its old speed with other applications. That is a Windows 98 system on a 150
Mhz PII processor.
I don't know what security we are losing by doing that, but the alternative
seemed to be to pitch it out the window. That system has a dial-up Internet
connection and so is not permanently connected to the Internet. You may want
to check for Norton or Symantec newsgroups or forums.