I realized when I said that about AVG that the problem probably didn't come and go via email, but the scan is the other part of the tool that is very good.
There are actually several very good free programs for this kind of thing, but you have to choose them one at a time, so if AVG doesn't help, look at the review sites and choose another. Use them one at a time--DON'T run these both installed at the same time. As a rule, competing programs don't play well together.