Yep--there's a step 3, but it has some side effects, so I don't post it
unless it is needed:
Download the Windows Installer Cleanup utility:
http://support.microsoft.com/?scid=kb;en-us;290301
(there's a download link in the above article.) Install it, then run it
from start, run. Scroll down til you find "Windows Defender Signatures...."
Remove just that, and nothing else.
Then retry updating. This isn't an instantaneous process--but between
915105 and the cleanup utility, you should be able to get new definitions
in, either via autoupdate in the program, or via Windows Update.