Windows Defender config disabled due to group policy on Home Premi

I'm running Vista Home Premium. I also use Sunbelt CounterSpy 2.5 for
anti-spyware and BitDefender 2008 AV for antivirus. Since Windows Defender's
real-time protection would conflict with CounterSpy, there's an option in
CounterSpy to disable Windows Defender. However, at some point when I was
using Windows Defender to check out the Software Explorer, I took a look at
the options and noticed that a) Windows Defender's real-time protection was
still enabled, b) it hadn't been updated in a long time and yet manually
updating it resulted in it saying no updates were available, and c) the only
option I can change is whether or not I run scheduled scans. The "error"
message at the top of the options window says, "Some settings are managed by
your system administrator."

I have checked the "Why can't I change some settings?" help link and the
only answer it offers suggests that someone applied a group policy to my
computer. But Home Premium doesn't have Group Policy Editor, and I don't know
the registry keys that would accomplish the same thing. I want 2 things: 1)
to have Windows Defender's real-time protection off, and 2) to be able to
configure Windows Defender at will and not via registry edit every time. A
3rd thing that would be nice is to understand how this happened in the first
place, but if I can at least get it fixed, that's not so important. (Although
it does keep me awake at night wondering if I need to upgrade to Ultimate to
have full control over my system in the future.)

Thanks for any suggestions,
Kira