I think your 5.0 build may have been corrupt.
What other security software do you have on your machine currently and previously including firewall?
Please try the following for your next uninstall/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:
Free –
http://files.avast.com/iavs5x/setup_av_free.exe Pro –
http://files.avast.com/iavs5x/setup_av_pro.exeAIS –
http://files.avast.com/iavs5x/setup_ais.exe 2. Download the Avast Uninstall Utility, aswClear5.exe
http://www.avast.com/uninstall-utility and save it to your HDD (it has uninstall tools for both 4.0 and 5.0).
You will need to uninstall BOTH versions.
3. Disconnect from the Internet at this time.
4. Uninstall Avast through "Add/Remove Programs" through Control Panel.
5. Boot into
Safe Mode (hit F8 repeatedly) and run the Avast Uninstall Tool.
6.
Reboot twice.
7. Clean your computer up (clean up cache, temporary Internet files, etc.).
8. Install the newest version of Avast and
reboot.
9. Get Internet access and update Avast definitions.
10. Register your copy or add the license key for Free, Pro or AIS.
11. Reset your settings, if needed.
Please let us know if this helps.