As YoKenny said support will finish very soon for 4.8, i suggest to download version 5 and the removal tool and save to your HDD, delet avast though add remove program's and reboot into safe mode then run the removal tool, once complete reboot again, now run a good cleaning program such as (ccleaner) or if you dont have, windows disc clean is better than nothing.
Now you can install your new version and then reboot, avast 5 does not need any key's to be sent to you, you just register though the GUI