The Fix
I had the same problem as you, and as you said, people would just say it's a beta and would add a ton of negative comments about Microsoft. I had searched Google for both the old MSAS and for the gcasInstallHelper.exe file, with no luck. Oddly,
after I had looked online, I looked on my computer. Sure enough, I had two copies of MSAS. They were both different sizes, but only one actually worked. The only problem is, I forget which one actually worked. I have uploaded the single gcasInstallHelper.exe file, along with both installation files to my site. Links are:
http://newgengamer.com/other/gcasInstallHelper.exe
http://newgengamer.com/other/MicrosoftAntispyware1.msi
and
http://newgengamer.com/other/MicrosoftAntispyware.msi
I would recommend trying just the gcasInstallHelper.exe file first, since it seems that it's the file Windows Defender complains about most, and it's only 30 KB. Simply place it in Program Files\Microsoft Antispyware and try installing Windows Defender. If you still have no luck, you'll have to reinstall MSAS. If your only options are Update and Remove, simply click next with none selected which will bring you to the install screen. Once that installs, install Windows Defender. Do not use the Add/Remove Programs thing to remove MSAS, since Windows Defender will automatically remove it.
That should fix it. Hope it works