For example, an Intel Core i7 processor takes just hours to crack a five-character password, but it takes more than 10 days to crack a seven-character password. That's why security experts recommend using passphrases instead of passwords. See Alex Wawro's password primer for pointers on building a good passphrase.

Stay updated: One of the easiest ways to prevent intruders from compromising your computer is to make sure that you're always running the latest version of all your PC applicationsincluding your antivirus program.

"Drive-by downloadsmalware that downloads to your computer when you click on a malicious linkoften work by exploiting known bugs in software," Harrison says. "These bugs are usually fixed in updated versions of the software, but that won't help you if you're still running the old version."

Prioritize accounts: You may not be able to remember complex passphrases for every account you have, and that's okay. According to Doug McLean, senior director of product marketing at McAfee's Global Threat Intelligence, the average online American has more than 100 accounts, not all of which are important.

Instead of creating different passwords for every account, create unique ones for only the important accountsemail accounts, online banking accounts, social networks, and other accounts that contain sensitive information. For relatively trivial accounts, such as message boards, it's fine to use an insecure, hackable password.