continue to use norton. have it scan the file after you have downloaded it and before you open it. you can choose to scan individual folders, so if you always download to one specific folder and then transfer the downloaded files later, you can tell norton to scan the lot when you have completed your download session.

grisoft.com supply a free for personal use package - avgfree - get the free package and the update to version 7.
install the free package first to get the registration number and then, when it has done its' first scan, install the upgrade. it will update automatically if you wish. avg works alongside norton and when one might miss a virus the other will get it.

in like manner, zone alarm free - zonelabs.com - will co-exist with norton firewall and both are keen to block intruders. it is worth putting up with a plethora of warning messages while they educate themselves as to what to allow and what not. investigate the options and experiment until you get exactly what you want.

spybot, adaware and spyware blaster will help to keep nasties off your machine and open source is a wealth of quality packages designed to help you keep clean.

microsoft anti-spyware beta1 is excellent and easily configurable. set package schedules to occur at different times from on another to avoid congestion.

good luck