Diallers are a problem which only affects internet users who have a dial-up connection. A dialler is a type of software mostly used by pornography vendors. Once the dialler software is downloaded and run, you are disconnected from your ISP and connected to another phone number. You are then charged for the use of this number. While diallers do not spy on you, they are malevolent in nature because they can run up huge costs for the victim. They usually connect to a premium-rate phone line, and some connect to international lines.
Most antivirus software can now detect diallers, so by keeping yours up to date you stand the best chance of avoiding this problem.