To register it on their website:
Go to Avast homepage http://www.avast.com
& under home user, fill out the form (the usual stuff: I've used Avast for some years now and never been nagged to buy the paid version, or any other Avast product)
They will e you a #; put that into the 'Registration' slot on the program registration:
Right click the blue tray "a" globe> Start Avast> Registration: enter # here...
Good for 1 year; then another free year w/new #, following above steps.
After installing Avast!
The default protection levels are "Normal"; make sure to move those to "High": right click the blue "a" globe in the tray> On Access Protection> on next window, tick each icon & adjust slider bar to "high' for each.
While your @ it, tick "Updating" > AVS Update.
On the upper right part of the Avast window (for the default skin) tick the box icon for 'Local Disks' depth of scan. Moving the slider right goes deeper, left less so. For the most thorough scan: slider farthest right & tick the small "scan archive..." box.
Below that is a folder icon where you select what disks ('c', 'd', etc.) or parts of your system you want scanned. Usually you'd want to select the 'c'.
When these are set where you want, move to the far left of the Avast window and click the ► arrow, and the scan will start.
'Move to chest' for any problem files found.
To reduce the need to 'interact' directly with Avast!:
Right click blue globe> Program Settings> Update (basic): set both updates to 'Automatic'.
In 'Confirmations' you can enable or disable what 'notice' pop-ups you want for behavior.