How does a computer virus enter a computer?

Answer:

from a website (use anti virus and anti spyware)
from an email (let the anti virus scan your emails)
from a pen drive/usb drive (scan with anti virus)
from an infected cd/dvd (use threat fire)
from the network (use a firewall)

First answer by Delltechie. Last edit by Delltechie. Contributor trust: 3064 [recommend contributor recommended]. Question popularity: 5 [recommend question].