Having two resident scanners installed is one too many and not recommended as rather than provide twice the protection it can cause conflicts that could leave you more vulnerable. So you really have to make up your mind which you are going to keep as you should only have one resident AV and uninstall the other.
What errors are you getting when you tried to a) closing it and b) when you tried to uninstall it ?
It sounds like you have somehow corrupted the installation if you can uninstall it from windows add remove programs.
Download the avast! Uninstall Utility, aswClear5.exe
find it here and save it to your HDD (it has uninstall tools for both 4.8 and 5.0). It has to be run from safe mode.
