I downloaded LATEST VERSION of Defender to my laptop and installed it successfully. Defender starts running.
Immediately, an Alert prompts for "update to latest definitions." I update. Alert still there. I update. Get bubble near system tray saying "No new definations available." Alert is STILL there. I do this 3 times. The alert to update definitions NEVER GOES AWAY.
I have followed suggestions in this site's knowledge base. I do not know how to mess with registry, but no other suggesiton helped.
I Uninstall, then re-Download, then reinstall Defender several times. I deleted the C:\Program Files\Windows Defender\MpAsDesc.dll file as suggested in knowledge base here. I update definitions, get "no new definitions...", and repeat process again...
The ALERT to update definitions NEVER GOES AWAY and still persists.

Thanks in advance,
Serob48

Lots of related posts. Plus, I Googled and found this:

http://www.technologyquestions.com/...

Life's more painless for the brainless.

When this has happens to me I install Defender then manually update the definitions just once from this link and Defender updates automatically afterwards. http://support.microsoft.com/kb/923159

Thank you both, Jennifer and Brim. Will try.

Thanks in advance,
Serob48

To Jennifer SUMN,

Follow up: The article is informational, and the writer of it succeeded in updating somehow. I followed that procedure several times with no clearing of the "update definitions" alert. It was worth a look just the same. -SR

Thanks in advance,
Serob48

To Brim,

It worked. By going to the Defender website, I found the UPDATE link, and manually downloaded. That satisfied Defender's request for updates and it seems to be running fine now.

Much thanks,
Serob48