Opening attachments from unknown senders is the riskiest thing you can do. Research shows that email attachments remain the number one means by which worms and viruses propagate.
Installing unauthorized applications such as file-sharing tools (Limewire, Azureus and other Bit Torrent clients) - Illegal downloads are against university policy and they, like email attachments, are simply another means by which "bad" files get on your computer.
Number three is disabling security tools. While trouble-shooting slow applications, many users will turn off their anti-virus and/or firewall. The problem is they forget to turn them back on!
While most people may know not to open email attachments, many don't realize that dangers can lie in the body of an email too. HTML mail or mails that contain embedded photos are just as dangerous. Embedded images and PDFs can contain malicious code that is harmful. So be sure not to open any unsolicited/suspicious mail.
Surfing questionable sites is always dangerous. You will find, more times than not, that porn, gambling and sites that host illegal content are the same sites that install malicious software on your computer.
Giving/lending passwords - Don't be too trusting of fellow students and colleagues. Keep your CGU UserName and CGU Password to yourself. Exposing it means you're exposing salary, banking, class registration information, etc.!
Browsers are quickly becoming some of the larger vulnerabilities in computing. Adware and spyware are written specifically to exploit Internet Explorer and Firefox. So avoid surfing sites that you don't already know and stick with the ones you trust.
Wireless networks are a huge risk because they are shared. The guy sitting across from you could be a hacker, stealing your password. Make sure you leave the firewall turned on and avoid sending passwords through the air. Only use encrypted wireless networks as they offer higher levels of protection.
Filling in web forms and registration pages - There may be nobody behind you watching you as you type. But that doesn't stop a keylogger (a program or device that logs all your key-strokes) from collecting your information. Try to keep all sensitive material on your own machine (the one that you maintain and protect), and keep it off those public computers.
Avoid social networking sites. Sites like MySpace and Facebook are a dream for thieves and stalkers. They allow anyone the ability to gather information about you that may aid them in stealing your identity. Think twice before you post any sensitive or damaging information on these sites.