What can I do ?   The system crashed when I was surfing on the internet .   Did you go to any illegal website ?   No , But does that matter ?   Yes , your computer can be easily infected by virus if you do that .   I see . I'd better never try .   That's wise .   Do you know what's wrong with my PC ?   One minute . Oh , yes , it was infected by a virus , and you had no antivirus software .   Is anti-virus software necessary for a PC ?   Of course . You'd better learn something about it .   I'm afraid yes . But what about the data I stored in the computer ? 