Have you ever received such message from a friend in YM? i.e. He'd say 'I just took an IQ quiz here.. pretty cool' then send a link.
If you've ever clicked on it, then your YM account security had most likely been compromised. A sure fire symptom of such hack attack is when friends tell you that your account is now sending above-mentioned message to them.
What to do to fix it?
1. The first thing you can do is to change your YM password. This works in most cases; But if symptom persists...
2. Use a malware software removal tool. There are lots of free ones you can use.
What's up, YM?