Disadvantages of computers:
Computers can make the mind lazy. You get used to not reading a book, or researching something at the library... Just a few key-strokes and you have your answer.
There are physical things like carpal tunnel syndrome and other repetititive strain disorders, and eye strain (often necessitating glasses).
It takes one away from socializing with others in person.
Spending too much time playing online games or chatting over the computer instead of reading books, taking walks, and doing homework.
Being on a computer too much can also lead to anti-social behavior and depression.
Violation of privacy, public safety
Impact on environment
Spam, hackers, viruses