Email is often used to transmit funny and entertaining programs, so it's not surprising that email has also become the number one way to get a virus or a broken account. Little falling sheep screensavers, funny animations of Santa or Tigger and simple games like WeaselWack and MindTeaser have all been modified to destroy security measures while they are run. The more popular the program, the more popular the malicious code hidden inside it.
You can't tell a clean version of a program from one that has been altered to include things like viruses and security-breakers. Treat executable attachments (those with extensions such as .exe, .vbs, .js, .hta, .pif and .shs) as you would hazardous waste material. Ask your friends not to email you files with these extensions.
Make it a habit to think twice about all attachments, particularly unexpected ones. Some viruses can email themselves to people in your address book using subject lines from previous messages. Even if a trusted friend emails you something, it doesn't always mean that they intended to or their account may have been compromised.
Equally important is that you don't indiscriminately download and execute program files from the Internet. Accept software only from an actual vendor. Download games and animations only from legitimate freeware and shareware Web sites that verify that their games are checked for exploits before they are posted. Any program not verified by a trusted source to be clean of viruses and security-breaking code is not worth the risk. Opening a program even just one time can cause permanent damage to your security.