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#11
i also stand with everybody here that is never run two AV simultaneously. One more thing windows add remove known to keep leftover of ununstalled software and AV's go very deep to the OS so when uninstalling AV always follow the guideline given by the vendor how to remove them properly because only they know how to remove them properly don't even use 3rd party uninstaller like revo to uninstall AV.
Removal instraction for MSE
How to manually uninstall Microsoft Security Essentials 2.0.1963 if you cannot uninstall it by using the Add or Remove Programs item
Removal instraction for Avast
avast! Uninstall Utility