I had that problem a couple of days ago but no problem now with the latest Windows Defender update:
Latest definition version: 1.99.165.0
http://www.microsoft.com/security/portal/Definitions/ADL.aspx
Choose the 64-bit definitions
That is not the latest definition.
I have 1.99.94.0
WD should update the definitions automatically.
If you are not sure if you have the latest definitions go to the Windows ball and click on it.
Type in "Windows Defender"
The link to Windows Defender will come up in the search. Just click on it to open Windows Defender
Click on the down arrow to the right of the Question Mark
Click on "Check For Updates"
If there is an update needed WD will download and install them.
You can check to see what versions of the Client, Engine and Antispyware definitions your version of WD has by clcking on the down arrow to the right of the Question Mark and then clicking on "About"
I recommend just sending along to MSFT the information that WD is requesting.