Experiencing the same behavior with an XP desktop at work and one of my users called me today about his XP laptop.

Great MS, first we had the Beta 2 ver of Defender erroring out on bootup and now definitions don't install. Sigh.

I'd say the best fix is to uninstall this POS program......

The posts in this list by "sjk" might make some sense to you:

http://groups.google.com/group/microsoft.public.windowsupdate/tree/browse_frm/thread/8990e0cb0b68985c/ecc4789bb1e4d938?rnum=1&q=0x80070643+defender&_done=%2Fgroup%2Fmicrosoft.public.windowsupdate%2Fbrowse_frm%2Fthread%2F8990e0cb0b68985c%2Fecc4789bb1e4d938%3Flnk%3Dgst%26q%3D0x80070643%2Bdefender%26rnum%3D1%26#doc_1b5ee63bd04d3473

Pretty strange.

About a week ago I disabled the "automatic" feature of the updates then tried a few times again to manually download and install the definition update - and had no luck (installation failed).

A week or so later I tried the manual update attempt from Defender again - but this time it installed as it should have. So now things appear to be functional.

I just tried this on the latest version of Defender and it still works.
Installed and downloaded the definitions and it's scanning.

More and more on XP/Vista (and I guess 2000 desktops as well although I don't use them that often).

I've been having to perform the following steps whenever Defender does not want to download definitions:

Start/Run/CMD
Surf to the Windows Defender directory in Program Files
type in MPCMDRUN -RemoveDefinitions -All
Net stop "Windows Defender"
Net start "Windows Defender"
Go to Windows Update and download the definitions. (Doing it through Defender the first time doesn't seem to work).


I just installed the latest Defender on 2000 and suprisingly it immediately downloaded the updates after install. Amazing.

Tried this last week and the modification and installation worked perfectly. Defender updated the definitions automatically and a full scan went without a hitch using the definitions of 18/7. Updayed today with definitions of 24/7, but now I can only carry out a quick scan- a full scan stops after a few minutes and I get a warning with code 0x8050800c. Rebooted- same result........... Ran Norton's Win Doctor, defragged......... still the same result. Full scans on XP using the same definitions worked flawlessly.

Hmmm, make sure your running the latest ver of Defender and check you event log for the error. It may show the file it gets hung up on.

IIRC, the quick scan only checks a few locations whereas the full scan scans the entire disk so there may be a file that's locked and it cannot scan.

Despite several cleanups, Defender updates etc I still get hangups when running a full scan. This file is the culprit:
HKCU@S-1-5-21-861567501-1993962763-839522115-500\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap\Domains\elitemediagroup.net

Have been unable to find it with regedit, though.

Right - now does anyone know how to get MSN Messenger 7.5 running on Windows 2000?

Well despite the OP being in 2006 this still works as I have just got Defender back up and running - belated thanks DosFreak.

heh. You can thank MS for that. They haven't done any development on Defender for a long time and probably never will. I'm guessing their OneCare service is the replacement.

I still leave Defender enable on my systems but it's probably not worth the cycles and memory it takes up.