Computer Virus

1.) Any program on your computer that reproduces its code
2.) Any program on your computer that completely fucks it up and disables its ability to work properly
3.) A said program that you probably got when you opened an e-mail you didn't know who it was from or downloaded something that you didn't know what it was
4.) Windows

Person 1: My computer keeps crashing, I think it has a virus.
Person 2: Did you open any emails you recieved from a stranger?
1: No.
2: Did you download any strange files?
1: No.
2: Are you running Windows?
1: Yes.