Norton is known to slow down your computer. I used to use Symantec Corporate Edition and I noticed no decrease in speed at all. There is no fix to the slowness that Norton causes. You just need to uninstall it and pick a different antimalware program. I suggest using
ESET Smart Security, which contains a firewall, an antispam module, and antivirus/antispyware. Another good choice is
AVG from Grisoft. There is both a free and paid version of this. In my experience, the AVG Internet Security program slowed down my computer a ton. After one day of using it, I switched back to the version that does not contain a firewall.