Can't install Avira--not allowed by administrator

My Avira Free was causing me a bit of trouble (Antivirus wouldn't run). I tried running AV as Admin, but nothing happened. After some effort, I uninstalled Avira, cleaned out the system as recommended. The installation gets to 60% or so; then I get a "not permitted" message, and the installation stops. I'm running under an Administrator account, and I'm running it as Administrator. What's wrong? I never had this trouble before.