For the most part it is just lack of knowledge.
Myth #1
"I just bought a new PC and it has Norton Security Suite so I must
be protected"
Truth: I have files that will crash Norton if you try to scan them, but they are
not malicious and won't harm your PC.
Myth #2
"I never visit evil websites"
Truth: Websites generate revenue from ads which come from other sites. A compromised
server can deliver bad things to your PC without you ever going there. Especially if you use
Internet Explorer.
Myth #3
"I only download files from my friends/people I know"
How can you be sure that file is safe? MP3's, JPEG's, PDF's can all be trojaned.
People simply don't know. When I sat down at a PC 10 years ago with Windows 98 and
AOL I had no idea what was out there. It wasn't until a few password stealers and one very
nasty Deltree program that I looked into securing my computer. I learned quickly not to click
on the latest greatest screensaver, mouse cursor changer, disk cleaner, super secret program of the day.
This is a great article for new PC users:
Infected in Twenty Minutes