Skype is no stranger to the messenger line of software and now version 4.0 has been released for Windows (XP/Vista). If you are using Skype, be sure to use the most current version as this sort of software you always want to be using the most current version of due to the security issues that typically crop up over time. Having an older version installed is only asking for troubles when it comes to security issues.
You can download the latest version here.