I just uninstalled and reinstalled my avast home edition because it was corrupted by a virus last week. Do I have to reregister or do I try to find my previous license key?
Also during the scan while reinstalling it found one file that was infected, which I placed in the chest. How do I deal with that?
Assuming your license didn’t expire (and, of course, if you can find it), just plug it into the new installation and it’ll work fine. If not, simply re-register online here which for the home version you can do at any time. I set up one mailbox in my mailer, “Product Registrations”, as a handy place to keep things like that if they’re needed again.
With regard to your second question, most of us recommend that you leave the suspect file(s) in the chest for a month or so, to see if any other programs complain about it being missing. After that time, you can usually safely delete it.