Dear Customer,
Thanks for your interesting in Microsoft Fix it Center(Beta). Based on your description, I suggest you try the below solutions:
1.
You might try this. Go to the main Windows Update window and
click on "Installed Updates" (on the left-hand side of the window, at the
bottom), and then remove "Update for Microsoft Windows (KB931099)" (only).
Reboot. Reboot again. Go back to Windows Update, and re-install the same
update, by clicking on the Check for Updates button.
This should essentially re-install Windows Defender on Vista.
2.
1 Go to control Panel and click on Administrative tools.
2 Click on services and a new windows will appear.
3 Look up for windows defender and right click on it, click on
properties and select automatic, it should be on manual, change it to
automatic and click apply then o.k. and will take you back to the
services windows.
4 Windows Defender should be highlighted and on the left hand side of
the services windows, should be 2 links, click restart.
5 Close all windows and open Windows Defender, it should work without a
problem and your popup window at start up will not be display any
longer.
Any question about this issue, please let know!
Sam