NOTE: Using avira registry cleaner may also delete avast related entries.
It will.
Fortunately you can select which items to delete; just don't delete anything from Avast. If you're unsure, post the entries concerned here.
Most free versions of commercial AV's have some feature or ability part-"crippled" or reduced. With some it's the detection or cleaning ability. With Avast it's the ability to schedule a scan. And a few other bits and pieces.
(One reason Avast is considered very good; the protection is excellent. In fact "advanced+" according to the latest AV-comparatives.)
A thorough scan will take a long time, it scans inside archives. These can be ignored (skipped) if desired, a virus inside a zip or rar can not activate unless the file is unzipped, at which point the on-access scanner should stop it.
Personally, I'd just do one thorough scan after first install, then a regular scan every month or so.
The scan needs to be (more or less) attended up to the first detection (if there is a detection) at which point the box in the alert window "do not show this next time" can be ticked, and the scan left to complete, while the user goes to bed, or work, or whatever.
You ca place an Eicar (simulated) virus near the beginning of the system folder on the C drive, that should speed the time taken for the first detection. You would need to pause the webshield to download it.
http://www.eicar.org/anti_virus_test_file.htm"A file is a decompression bomb" is alarming terminology. It just means the file is packed using an unknown and/or high compression algorithm. (Sounds fancy, don't it?
). Don't be alarmed, it is no indication the file contains a virus, just that it can't be scanned.
Other similar errors you may get are "...the file is password protected". Once again, don't worry. This could be from the quarantine of another anti-malware scanner, or some program that uses encryption to store some of its data. Moving the column header of the scan report to see the full name and path of the file will usually reveal whether it needs further investigation.
And if you don't know in regard to a particular file, you can always ask here.