Viruses are spread by various means such as pen drives, Internet, emails, and sometimes by Bluetooth devices.
Both Trojans and worms are really deadly and they really cause serious harm to the system.
Each and every virus has its own way of damaging a computer. To help protect a virus, you need a software called the Antivirus. There are a lot of Antivirus softwares available, some in free edition too. Check out the Avira Antivir Free edition which is a good Antivirus.