1. Download MSE from the website and save the file on the Desktop.
2. Open Command Prompt through RUN or Start Search by typing CMD hit enter. If you are using Vista right click on CMD and select "Run as Administrator".
3. In the Command Prompt console type: CD %USERPROFILE%\Desktop <- Hit ENTER
4. Type: msse <- then hit the TAB button.
5. Depending on your Operating System, after hitting tab it should either appear as any of the three below:
mssefullinstall-x86fre-en-us-xp.exe
mssefullinstall-x86fre-en-us-vista-win7
mssefullinstall-amd64fre-en-us-vista-win7.exe
6. Now put a space in between the whole line and put /U as seen below:
mssefullinstall-x86fre-en-us-xp.exe /U
mssefullinstall-x86fre-en-us-vista-win7 /U
mssefullinstall-amd64fre-en-us-vista-win7.exe /U
7. Hit ENTER and this should uninstall Microsoft Security Essentials.
8. After the uninstall you can now attempt installation. It would be best that you restart your system first after the uninstall.
note: i didnt find this myself