Bitcoin may still be a virtual unknown quantity for most people, but the digital currency has not escaped the notice of attackers, many of whom are turning their attention to finding ways to use the system for their own gains. The attacks against Bitcoin exchange Mt. Gox and hack of Instawallet this week are the latest evidence, but now there is a piece of malware in circulation that is using Skype as a spreading mechanism and then using infected machines' processing power to mine Bitcoins.