First, you should upgrade or do a clean install to the latest version of 5.0.594 since the support for 4.8 will end at the end of this year. You can either do an upgrade, or since you are already having a problem with doing a boot scan, try a clean install.
Do do an uninstall of Avast and
CLEAN install:
1. Save a copy of newest version of Avast (
5.0.594) for the version you need and save it to your HDD:
Pro –
http://files.avast.com/iavs5x/setup_av_pro.exe2. Download the Avast Uninstall Utility, aswClear5.exe
http://www.avast.com/uninstall-utility and save it to your HDD (uninstall version 4.0 from the drop down menu).
3. Disconnect from the Internet at this time.
4. Uninstall Avast through "Add/Remove Programs".
5. Boot into
Safe Mode and run the Avast Uninstall Tool.
6.
Reboot twice.
7. Clean your computer up (clean up cache, etc.).
8. Install the newest version of Avast and
reboot twice.
9. Get Internet access and update Avast definitions.
10. Register your copy or add the license key for Pro.
11. Reset your settings, if needed.
On-line registration is very simple and it's the preferred method for all users of Avast Antivirus 5.0 installed on a computer with standard Internet connectivity. To perform on-line registration, follow these steps:
http://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=459Please post your results. Thank you.