The license key period is counted by the date of its generation and not the date you've added to the program. So, you can add your bought key right now.
About the virus, can you say what is the infected file name, where was it found (C:\windows\system32\infected-file-name.xxx)?
What avast! version and virus database are you using? (see About dialog of avast!)
Also, the safer is send the file to Chest than direct removing (deleting) it.