See this post, maybe you are lucky.
Subject: 0x80004003. Invalid Pointer
2/19/2006 12:45 PM PST
By: lou g
In: microsoft.private.security.spyware.general
Subject: Windows Defender error message
2/27/2006 6:14 AM PST
By: ffjuser
In: microsoft.private.security.spyware.general
Click the down arrow to the right of the Windows Defender ? (Help options)
and click About Windows Defender. Post the version numbers of WD, engine and
definitions. Also, are you a Windows 2000 or XP user. I have a suspicion
that you are back level on Windows Defender.