Any program that does live scanning will slow down your computer slightly because it does pretty thorough checks on new files and on files as you access them. However, antivirus software is still important because even trusted sites can be compromised and the virus can spread on your system quickly.

One of the things I like about the Avast software is that it can be configured to run the full scan when the screensaver is running so it doesn't affect performance when you are working.

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The best way to learn anything, is to question everything.