Microsoft has released a fix after Security Essentials and Windows Defender stopped functioning on Windows 7 and 8.1. Users began reporting problems on AskWoody on March 19, with no error notification until they attempted a manual scan.
Upon scan, Security Essentials returned the message: “Your PC couldn't be scanned. This program's service has been stopped. You can start the service manually or restart your computer, which will start the service.”
Naturally, restarting the computer did not solve the issue as is claimed. This left users without virus protection for some time, but a definition update is now available. You can grab 1.289.1588.0 manually here, which should fix the issue.
Microsoft was not officially clear what caused the issue, but a source familiar with the matter did talk to ZDNet's Mary Jo Foley. The problem was reportedly introduced with signatures in definition 1.289.1521.0.
WSUS Issues Linked?
The company hasn't been particularly communicative with its users about this issue, which raises concerns about its promise to support Windows 7 until early 2020. However, it did at least act quickly, with everything resolved within 24 hours.
Unfortunately, it seems these signature issues may have also caused problems with Wsus a couple of days ago. Users were reporting sync errors with the update service, which were quickly resolved when all definition updates were removed.
Microsoft is yet to confirm if that was the case, but it seems like a plausible theory. Either way, both issues appear to be resolved with only minor inconvenience.