Skype for free or cheap VoIP phone calls over the Net via a Windows computer, and you haven't updated your Skype software lately, you ought to - there was a critical security hole which version 3.6 of the Skype software for Windows, an update issued in mid November, has now fixed.
With previous versions, there was a (it seems not at all well publicised) risk that if you visited certain malicious websites your computer could be infected with malware - viruses, spyware, etc, or control of it taken over by bad hackers.
So if you have a version of Skype for Windows that's below v3.6, you probably ought to upgrade Skype to the latest version as soon as possible. There are also other Skype issues to watch e.g. with trojans pretending to be Skype plugins.
Via Heise Security, which has more info on the security vulnerability.