Heidelloon,

Are you saying you have this Trojan, if so could you provide some more info on it, such as the file name and path.

If you just worried about getting the Trojan and would still like to chat on Yahoo messenger, i suggest turning your firewall settings up high, and keep avasts ‘network shield’ open, also turn avasts settings up to high, this should stop the Trojan.
Only other thing you can do it stop using Yahoo Chat server for a while until the problem blows over.

–lee