Office Enterprise 2007 trial not signed?

I downloaded the Office Enterprise 2007 trial and when choosing to run it after the download finished, I got a prompt that the publisher could not be verified, so the trial seems it doesn't have a valid digital signature:

The file was downloaded here http://www.microsoft.com/downloads/details.aspx?familyid=2D1189BF-D86A-4ACF-9DCC-4D61F500AD6D&displaylang=en and everything installed fine, though it seems kind of strange for Microsoft not to sign their own code. Anyway, this warning was received on a Windows XP SP2 system with IE 7.0.5730.11 installed.