Defender service on Vista x64 won't start

Just installed Vista Ultimate 64-bit version a few days ago. Defender was
working normally. A day later, possibly coinciding with having installed the
updates from Windows Update, the Defender service will not start
automatically or manually.

I tried every "fix" I could find:

- Client for Microsoft Networks is installed
- I unchecked all the boxes on Windows Update (WU) settings, stopped the
WU service, deleted the Software Distribution folder, and restarted WU
- I attempted to uninstall Defender to reinstall it (already downloaded
x64 Defender) but it doesn't show up in the Programs (to uninstall) list or
the Windows features list. I tried a command line uninstall but it give the
same error as when I try to manually start the service. Unable to initialize.
- Tried to install the one I downloaded over the current one but it tells
me it's already installed and exits.
- I do not have OneCare installed and never have because apparently it
doesn't work on Vista64.
- Tried restoring the default signatures from the command line
- Tried manually downloading the newer signatures and installing them

Defender still won't start, unable to initialize and the event viewer says
it can't find the specified path. I checked the registry to find out which
path it's trying to launch Defender from and it points to Program
Files\Windows Defender, I checked the Windows Defender directory and all the
files are there. There's a second Windows Defender directory under the
"Program Files (x86)" directory but none of the registry entries pointed to
it.