Viruses are lurking everywhere. The borrowed CD is infected with a Trojan horse that enables others to remote control your computer, a downloaded screen saver stores dangerous viruses beside the image of the romantic sunset and emails can hide worms - even without an attachment.
While surfing on the Internet your computer gets infected with viruses that are hidden in ActiveX-applications. Seemingly harmless emails corrupt your computer through polymorphous viruses, which can change their form constantly. A virus lurks in a compressed file and gets activated only when the file is decompressed.
A wrong mouse click can install backdoor software that allows others to access your computer.