i just tried to install MSE on my daughters laptop and each time i got a corrupt file error message ,after a quick google it is a known issue to microsoft for the english x64 version only.
there is a workaround that works and to do so download and install a different language x64 then do a registry hack.( i used spanish)
1. click the "windows key" and type in regedit
2. run regedit as administrator to ensure the proper actions are taken.
3. follow this HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft Security Essentials
(edit: or microsoft security client)
4. There will be three files in this area.
5. Right-Click on (Default) and select Modify
6. Under Value data:, type in EN
7. Do #'s 5 & 6 again for Market
8. MOST IMPORTANTLY, RESTART COMPUTER AS SOON AS POSSIBLE AFTER FOR AFFECTS TO TAKE PLACE.
respect and thanks to boblynch for his post here...
Corrupt file installing Security Essentials - Microsoft Community