I’m now a user of avast 4.8 professional edition and acquired it online, but I found out that there are corrupted files in my system and I have to recover lost files. Can I reinstall and register the product with the same license key after I recover my lost files? 
If you have saved the email with your Pro license you can use that yes. provided of course where you acquired it on-line is legit, as there are fraudulent sites selling avast. Where did you get yours ?
However, if these files are avast files first try a repair of avast. Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow.