Should i enable all shield in antivirus?

Hi,
I am using avast antivirus, and using file shield protection only. If i enable, the web shield and mail and other shield, avast open some ports . I do not know, whether the shield will scan only my outgoing email and incoming email only. Should i enable these shields as normally google sends spam mails in separate enclosure. Please advise, the antivirus, use about 10 to 15 ports to the minimum. This i understand, when i went to the settings of the concerned scans.
Please say , the background scan also takes so much of cpu resources, that even the computer sometimes hangs with no movement whatsover.
Any how i have to have a antivirus. My question is : is it enough if file protection is enough, as all the mails are filtered by respective service mail providers

would anybody see this thread

I use Avast free with just the basic 3 shields - File System Shield, Mail Shield, and Web Shield.
I don't use any of the optional tools, i don't want them.
I don't have problems with this.

cpu resources, that even the computer sometimes hangs with no movement whatsover.

I don't have a resource/hang problem with Avast.

Please advise, the antivirus, use about 10 to 15 ports to the minimum.

I don't know what ports Avast uses.
If you can tell me what/how to look at this, i'll check what i see on my PC, and post back with that info.
I use the built-in Windows firewall.
If you can add screen prints of what you see, that would help.
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