There are obviously measure in place to prevent people continually using the trial version.
So that may well be what you are bumping into; as for another tool to erase that record, I rather doubt that as it would mean the measure to prevent this would be pointless.
Remember if you purchase avast Internet Security, you have 30 days in which to seek a refund if it isn't compatible, etc. Other than this I don't know of any other means to get round this issue.