Somehow I doubt that you dad formatted the computer otherwise it wouldn't turn on, unless he also reinstalled all the programs.
What is your OS ?
Are you getting any error messages ?
Is there anything in the avast! Log Viewer ?
Do you have any other security programs installed AdAware, Spybot, SpySweeper, prevx, etc. ?
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. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot. - watch for any warnings when you reinstall.
It would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall.