There’s a new trojan going around that is disguised as a Skype plugin called Skype Defender. If you install it, it will take your username and password and send it to a hacker.
It seems it only affects Windows users, and you recognise it by looking at the login button on Skype (the real one has a red bordeR). This document from Skype shows exactly what it looks like. It gets installed by a program called 65404-SkypeDefenderSetup.exe.