Publisher Couldn't be Verified

The publisher of winscp434setup.exe couldn't be verified.

During download.

Windows 7
IE 9

Microsoft Security Essentials
Definitions created on: 8/22/2011 at 3:45 AM
Virus definition version: 1.111.433.0
Spyware definition version: 1.111.433.0

I'm not sure what Microsoft Security Essentials mean by the error message. But definitely, there's no way it can verify "the publisher" as we are not registered anywhere.

Edit 2021: Since 4.3.6, WinSCP binaries are signed with publisher certificate.
https://winscp.net/tracker/505

It's actually IE that brings this up. Potentially the file may have downloaded if you chose Save As.. and picked a place to save it rather than just Save when prompted.