Another method of infection is downloading games, sharing files with friends or playing MP3's on other peoples computers. An alarming number of them didn't know they have to update their virus protection. Many have been using the original program that came with their computer 2 years ago. At this point it is necessary to visit the room and try to remove the virus. Some are very easy to remove, and some take a person that knows what they are doing.
Some viruses ruin their data so badly that it requires reloading all the software on the machine.
This takes time and all can be avoided with a little education. Hackers can and do infect insecure computers. There are all kinds of hackers out there that want to prove how good they are or want to settle a score. They can attach a file to your computer and trigger it later to participate in a flood of access requests to a web site making that site stand still. That is called a denial of service attach.