Hi,
I fought this issue for weeks upon weeks upon weeks until I had no space left on my arms to cut anymore :d. This is how I fixed it:
Completely uninstall anything having to do with Annihilus or D2 on your hard drive. Next Delete all accounts of Diablo 2 or Anni from your Registry (Be VERY VERY careful with this step and only make sure you are deleting things with Anni and D2 in the name.)
Next, Fresh install Diablo 2 using the site or there is another thread on here for an alternate DL.
Restart your computer.
Set D2.exe and Game.exe to run as administrator
Restart your computer again.
Follow the DEP guide making sure there are exceptions for the updater/launcher, D2.exe, and Game.exe
Restart your computer once again.
Now, open the launcher and update. If the c000005 error doesn't go away, Restart one last time.
I know all the restart are very very annoying, but for some reason I found if you don't restart after each step it will never work.
Hope this helps!
PS your sig is sooo hot