People somtimes spend all their time secluded in theirs rooms on the computer.
Peadophiles on chat rooms
Easier to copy peoples homework as you can sometimes find the sources online.
Illegal downloading can ruin businesses
Details such as bank records, can be hacked into, and thus your personal information is no longer as safe as it used to be.
If you have little knowledge of them, it is easy to get in a lot of trouble and quite literally destroy your computer.
Depending on the OS and software/hardware installed, they can be difficult to maintain.
If you have the internet and don't have an Anti-Virus program, or at least an Anti-Spyware program, surfing the web can ruin your day.