i can't update my windows defender's virus definitions!!

today, when i opened the windows defender i saw my PC status at risk and when i tried to update it's virus definitions i couldn't complete, it says network connection but i could connect to website download anything without any interuption. i even tried to update virus definitions through windows update control panel but it was of no use. i had installed MSE on windows 7(i have a dual booting) .did this action affect my windows 8 OS and plz tell how do i get rid of it

Try downloading the latest definitions directly from Microsoft and do a manual install:

Install the latest Windows Defender definition updates - Get the latest definitions - Microsoft Malware Protection Center