This is Mike from the Norton Authorized Support Team responding to your posting.
Norton 360 is a comprehensive internet security suite that provides real-time antivirus, anti spyware, botnet protection, a full featured Firewall and many other features to keep your computer safe.
To answer you question about installing other internet security and antivirus applications, you can install some of these applications alongside Norton 360 if you wish, as long as they are not running in real-time. The rule of thumb is that you never want to have more than one antivirus/internet security application running at the same time or they can conflict with Norton 360 and leave your system vulnerable to infection. You should also make sure that you have applied all Windows security patches and make sure that both the Windows Firewall and Defender are disabled, as Norton 360 is handling these features for you, and if they are enabled they could also cause conflicts.
The most important steps you can take in securing your computer against threats is to practice safe browsing and to make sure that the Auto-Update, Antiphishing, Norton Safe Web and Identity Safe features are enabled. For more information on safe web browsing,etc, please read the following document:
Norton guide to browsing the web safely
You should also schedule "Comprehensive System Scans" at a time when you are not using your computer
If you are using version 1 or version 2, I highly encourage you to take advantage of the free update to Norton 360 version 3, which contains many enhancements and new product features. Norton 360 version 3 also allows you to scan in Windows Safe Mode, and some infections can only be detected and removed in Safe Mode.
Please NOTE, that while this process will update you to the latest version of Norton 360, it will not extend your subscription.
If you have not already updated to version 3, and you have a valid subscription, please visit the Norton 360 Update Center to install the latest version.
Norton 360 version 3 update
When the update is complete, launch Norton 360 version 3, mouse-over the "PC Security" bubble and choose "Check for Updates." This will not only download and install any available updates, but will also invoke the new "Pulse Updates" feature which will automatically check for and install definition files in the background every 5-15 minutes, so that you are always protected against the latest threats that are discovered.
Let me know if you need additional assistance.
Norton Authorized Support Team