Are Viruses, spammers and suchlike able to use Skype?

What you are using is a VoIP application. Any application can be exploited if it is connected to the WWW.

For the most part, VoIP is still misunderstood and the exploitation has yet to actually begin. The Telecoms are very afraid of such nonsense as FREE Phone Calls. While Skype is not free, there are some that are truly free.

Let us look at the fail point, you pay (X) for a service, that means that monies were transfered somewhere. Once you put the money wheel on the Internet, it lures the bad peoples in to try and garner that data for their own nefarious means.

Even just the Data is sometimes enough, (see China Listens to Skype). In that scene the Chinese government was found to be using some slick bots to 'tag' conversations and then monitor them.

The only risk to the PC at your desk is the last backup that you did not make. A good backup routine and let the thing crash hard as it wants, let any hooligan get any data they want, after I replace the system with the backup and plug any holes that existed, the better off we all are.

Follow these steps on this link.