Why can't I uninstall Avast Free Antivirus? It always says "there is an error" that occurred and doesn't uninstall the product.
Did you exit the application first?
Try with Revo Uninstaller.
The portable software program that is compatible with Microsoft operating systems from Windows 2000 to the latest Windows 7 Beta (including 64-bit editions of XP and Vista) can be run right after download. It will scan the computer system for security software programs and display all that have been found in a list. It is possible to remove all or only selected ones from the system.
Avast has its own dedicated uninstall utility called aswClear for removing Avast software from your Windows PC computer.
Instructions For Avast removal using this utility can be found here.