I used AVG for a long time without any issues, but when I had my computer repaired (non-virus thing), the computer guy put Avast on. He says he runs it on all his computers and recommends it to his customers. He says its the best out there. It's also free.

I have newer Excel but often have to "save down" to share documents with clients who have 2007. I've experienced no problems doing that.

YMMV.