David, the most general advice is see whether you are able to update via
Windows Update.
If that fails, you need to troubleshoot Windows Update.
If that works and you are not on a managed, corporate network, use the
resolution steps in:
http://support.microsoft.com/?kbid=915105
If you are on a managed, corporate network, updating within the application
is handled via AutoUpdate. Windows Defender signature updates can be made
available via WSUS if the WSUS administrator approves them. I have seen a
post from a Microsoft staffer stating that this is true for SMS as well, but
I'm not clear on the details.