Most viruses are installed during the night while your computer screen is turned off and your computer NOT active. They 'hackers' know that your not there so your not going to notice it happening to stop it. They can get into your computer and make norton and other AV(Anti Virus) programs to not notice it or not detect it as a threat. This can be very annoying especially if it is a sleeper, where it comes out after a period of time, and can really screw up your computer and make it go a lot slower.

So if you weigh the slow down of the AV running in the background to spyware and viruses destroying and corrupting data. Having that little slow down from the AV is much better then your data that you find usefull and may NEED later being deleted or corrupted so that you can't get into it.

I have the following AV ( Anti Virus ) programs:

Norton
a² Personal Edition
Panda Platinum Internet Security
avast! Virus Cleaner Tool

I think that is all the AV ( Anti Virus ) programs that I have.

I also suggest you getting at least ONE of these if you do not already.