Microsoft Security Essentials reporting false positives in the Bitcoin blockchain, constantly notifying users.
https://answers.microsoft.com/en-us/protect/forum/mse-protect_updating/microsoft-security-essentials-reporting-false/0240ed8e-5a27-4843-a939-0279c8110e1c

So it's confirmed, thank you. Of course it's just a text signature, but still quite an annoying prank.