It is not recommended to use a third-party uninstaller like Revo for ANY security software, especially an antivirus because drivers and registry keys are often left behind. Instead, you should use the vendor's uninstaller tools to uninstall the software. Avast has it's own Uninstall Utility Tool for this purpose.
1. Download Avast Uninstall Utility Tool, aswClear5.exe http://www.avast.com/uninstall-utility
and save it to your HDD (it has uninstall tools for both 4.0 and all Avast products).
2.. Download the Avast product you need.
3. Disconnect from the Internet at this time.
4. Go to Control Panel and uninstall Avast through Add/Remove Programs if possible and reboot.
5. If Step 4 fails, boot into Safe Mode
(hit F8 repeatedly) and run the Avast Uninstall Tool. Uninstall all Avast products/versions you have on the machine at this time.
7. Install the newest version and product of Avast and reboot
8. Get Internet access and register your copy or add the license key.
9. Update the Avast definitions.