Installing AVG on Mac

Hello Sandra,
Thank you for contacting us with your concerns. You seem to have disabled the installation of apps from third party sources. Please change your security settings as described below to allow the installation:
1) Open OS X System Preferences > Security & Privacy.
2) On the "General" tab click the lock in the lower left corner to unlock the general preference pane.
3) Under "Allow applications downloaded from:", select the "Anywhere" radio button.

Please, feel free to let us know if you require any further assistance. Thank you.

I have downloaded AVG free edition on my laptop with no problem.
Just downloaded onto my desktop and get the following message:

“Install AVG AntiVirus” can’t be opened because it was not downloaded from the Mac App Store.

Your security preferences allow installation of only apps from the Mac App Store.

“Install AVG AntiVirus” is on the disk image “avg_avc_mac_all_ltst_151.dmg”.
Safari downloaded this disk image today at 11:20 from www.avg.com.

The above did not appear on Macbook..........do I have a virus which will not allow antivirus download on desktop, if so how can I download and clean Mac?
 

Hello Sandra,
Thank you for contacting us with your concerns. You seem to have disabled the installation of apps from third party sources. Please change your security settings as described below to allow the installation:
1) Open OS X System Preferences > Security & Privacy.
2) On the "General" tab click the lock in the lower left corner to unlock the general preference pane.
3) Under "Allow applications downloaded from:", select the "Anywhere" radio button.

Please, feel free to let us know if you require any further assistance. Thank you.