Norton is an alright program, but you have to have a front line of defense, to stop stuff from even GETTING on your computer. A firewall that is set correctly will work wonders and keep ALOT of stuff away from your PC. I use ISS Black Ice, a great firewall that is very customizable, but is more for advanced users since it can be confusing. If you want a great free firewall, get the free personal firewall from www.sygate.com
Firewalls will stop things like TROJANS and SASSER from getting into your computer.
The Quarentined files are probably safe from harming, since a virus does just what the name means, and makes copies of itself so to speak, a quarentine is 'medicine' in a way and should stop it from replicating itself and doing a lot more damage. A lot of things I have found are not cleanable, so just keep them in your quarentine or kill them manually.
If you get this many viruses, you not only should have symantic, and get a firewall, you should have symantic running on the system tray (the clock area) at all times, this should scan any incoming files for viruses, and if there is any, it will quarentine the file before you even become infected.
If the problem is really severe and the viruses are doing damage even if as many as you can find are in quarentine, sadly you might have to format your hard drive(s) and reinstall windows, or use the system restore disk included with some PC's and Laptops. I got a new hard drive for my B-Day (Getting it tomarrow) and I have to format, but I am backing up files right now, it is tedious. IF you do have to format, I suggest getting
Winrar and putting your files into a compressed folder (.rar) to save space.
Best of luck my friend.