Tech Republic has an article on the 10 things people do that hurt their computers. By following the link, you can read the full article.

Some of these are obvious, some are not.

#1: Plug into the wall without surge protection.#2: Surf the Internet without a firewall.
#3: Neglect to run or update antivirus and anti-spyware programs.
#4: Install and uninstall lots of programs, especially betas.
#5: Keep disks full and fragmented.
#6: Open all attachments.
#7: Click on everything.
#8: Share and share alike.
#9: Pick the wrong passwords.
#10: Ignore the need for a backup and recovery plan.