They will download any program they find, whether or not they have any use for it, meanwhile tying up the line. A Trojan Horse is an innocuous- looking origran that when run reformats your harddrive, destroying all your files. To protect yourself against this, ask the store where you bought your computer, or an experienced computing friend, for a Trojan detector program. Put every program you download through this before you run it. A virus program is a relative of a Trojan horse, but is usually inserted in a proven program.
Users are often less suspicious of well-known programs.