Well you say the Repair and Downloading the full virus definitions database didn't resolve it. The next step in progression would be a clean reinstall, but you say you want to avoid that. Which leaves the status quo and use the full virus definitions database database.
Having gone over the whole topic (as primarily I was only responding to Asyn's post), are you allowing the program to auto update the virus definitions ?
If so are you getting any errors with that also or just on manual attempts ?
What does your defs folder look like (the sub-folder names are similar to the virus definition version), a little earlier mine was like this (see image) ?