Windows Defender Problems

Using a laptop running Windows Vista, when trying to change my startup
programs, I get a message saying, "Windows Defender is turned off" that
won't let me change the programs until I click "Turn on and open Windows
Defender." When I try to turn it on, I get an error message "Windows
Defender encountered an error 0x800106ba. A problem caused this
program's service to stop. To restart the service, restart your computer
or search Help and Support for how to start a service manually."

I tried using the information from this page} (http://support.microsoft.com/kb/931849), but none of the steps have worked.

NOTE: My error message is neither of the ones listed on that page, but the number is the same.

Step 1: I did, but I still get the error message.

Step
2: Windows Defender is not in my list of programs to uninstall, but it
is listed in the start menu. I still tried reinstalling it but got the
message, "You do not need to install this software because Windows
Defender is included in Windows Vista. You can access Windows Defender
from the Security section of the Windows Control Panel."

Step 3:
Again, Windows Defender is not in my list of programs to uninstall. I
tried skipping to d (using the Windows Installer CleanUp utility) but
when I search for "programs and features" there are absolutely no
results.

Step 4: The first two file names succeeded, but at the
third one it tells me "The module wucltui.dll failed to load.... The
specified module could not be found." Fourth works, but wuweb.dll
doesn't. The next few work until I get to initpki.dll, and the last one
does work.

I restarted the computer in the hope that something might have changed, but to no avail. Any advice?

NOTE:
From reading other posts, it seems that most people get this message
every time they turn on the computer, but I never get it unless I try to
turn it on manually.Hello. Glad to have you here at the forums.

Have you had any blue screens or crashes or other system damage lately - such as a virus infection?

Any recent install of Windows Updates that you feel were not installed properly.My computer overheats a lot (multiple times a day--and yes, I am keeping
it on flat tables), but otherwise no problems. I've caught a few
viruses and such with my virus scanner, but they had never caused any
noticeable harm. I'm using Microsoft Security Essentials, and there are
three items I've removed, but they only date back to last month, and I
had encountered this problem before then.

As for updates, I
wouldn't know how to how to check whether they installed properly.
Everything seems okay, and Windows Update doesn't have any important
updates for me to install.