Many members (of whom I am one) here use AVG Free Edition, although there are several other good free AVs available. In addition to any AVG, you should have a resident Firewall and several anti-spyware applications on your hard drive.
Links to member-recommended freeware can be found here:http://www.bleepingcomputer.com/forums/topic3616.html
I would suggest you proceed on the premiss that your computer has NOT been protected during the summer, and spend some quality time downloading, installing, updating, and running your anti-malware applications.
What? You want to leave the dorms? "Flush twice, it's a long way to the cafeteria."