1. What did you try as cleaning temp files followed by a defrag I would think would take more than an hour and a half.
2. Which would probably be the access problem if it doesn't exist.
I don't know if this would work but you could 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 the repair doesn't work try, uninstall, reboot, run the uninstall utility, reboot, install the latest version you downloaded (see below), reboot.
I may also be best to ensure all avast registry entries are gone. Download the avast! Uninstall Utility,
find it here and save it to your HDD so you can find it later.
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.