Viruses, spyware, malware, port scanners, malicious hackers - surely there's no way your computer can be safe these days? Well, there are several things you can do to keep your computer safe, even if you aren't willing to spend money on it.
Protecting against viruses: you need antivirus. Actually, on Linux you generally don't, because there aren't many viruses around for Linux (due to the limited number of people running it). Windows users, however, should definitely get some antivirus protection, and I'd even start recommending it for Mac OS users, as Apple Macs become more popular. If you're willing to pay, I'd recommend either Grisoft or McAfee. But this article is aimed at poor, impoverished students, so free is the way forward. You can get Sophos anti-virus from the university, download AVG Free Edition, AntiVir PersonalEdition Classic or Avast! anti-virus. Note that I have given them in the order they were ranked by PC World. Also, not all these programs are available for Mac OS. Just run the program you've downloaded, and follow the instructions, to install your antivirus. It should keep itself up to date. You should only install one antivirus program at once.
Firewalls protect you from people and programs trying to force their way onto your computer directly when you're on the internet (and if your computer is plugged into the college network, then that's all the time it's on). Mac OS X has a built-in firewall, as do Linux distributions. See the documentation or support forums (or Google) for how to enable these. On Windows, you can download Zonealarm, Comodo or Outpost (requires registration) for free to get a firewall. You should only install one firewall program at once.
Mainly for Windows users. To protect yourself against spyware, you should run some anti-spyware software. Two of the best (free) anti-spyware programs are Spybot Search & Destroy and Ad-Aware. I recommend you install both, and run them regularly (once a week or fortnight, ideally - or whenever you feel like procrastinating, since that's likely to be frequent!). AVG also have a free anti-spyware tool.
Updates! Keep your computer updated. Turn on automatic updates on Windows and Mac OS, and on Linux if your distribution has that option (Ubuntu, for example, should flash a warning when there are updates to be downloaded, and you can click the icon to download and install them).