Up-to-date security software is an essential part of keeping your PC safe.
There are two main reasons why your software may not be able to update:
This may mean you can’t access some or any websites, or even when you can access the Internet, your security software won’t update.
What you need to do
There are three steps you can take that might get your antivirus or antimalware software working properly and updating:
You can also try searching the Microsoft Community Forums for help.
Run the Microsoft Safety Scanner
If you can’t connect to the Internet, or there is a problem updating your definitions, we recommend you:
Go to another PC that isn’t infected
Download the Microsoft Safety Scanner onto a USB flash drive
Run the Microsoft Safety Scanner from the USB flash drive on the infected computer
Uninstall then reinstall your antivirus software
To rule out a problem with your security software, you can try uninstalling it then reinstalling it.
You can learn how to do this in the article Uninstall or change a program.
Use Windows Defender Offline
If you’ve tried the Microsoft Safety Scanner and uninstalling then reinstalling your antimalware software and you’re still having an issue, we recommend you download and run Windows Defender Offline.
Windows Defender Offline is a standalone tool with the latest antimalware updates from Microsoft.
It’s not a replacement for a full antivirus or antimalware solution that provides ongoing protection. It’s meant to be used when you can’t start or scan your PC because infected malware infection is stopping your antimalware software from working.
Before you begin you will need:
A PC that is not infected and is connected to the Internet. You will use this PC to download a copy of Windows Defender Offline
A blank CD, DVD or USB flash drive—use this to run the tool on your infected PC
Follow these steps to use Windows Defender Offline:
Use an uninfected PC to download a copy of the tool from here: Windows Defender Offline
Make sure you download the right version for your infected PC. For example, your desktop PC has been infected with malware. It is running a 64-bit version of Windows. Your friend's laptop, however, is not infected, and so you use that to download Windows Defender Offline. Your friend's laptop is running a 32-bit version of Windows, so when you download the tool, you choose the 64-bit version, because that is the version that matches your PC.
Install the tool on a blank CD, DVD, or USB flash drive
Insert the CD, DVD, or USB flash drive into your infected PC and run the tool
Let the tool clean your PC and remove any infections it finds
After running the tool, make sure your antimalware software is up-to-date. You can update Microsoft security software by downloading the latest definitions.
For detailed instructions on using Windows Defender Offline, see the Microsoft Security Blog post "Microsoft's Free Security Tools - Windows Defender Offline".