Short answer : Yes .. you had better use a firewall (with anti-spoofing technology) and antivirus 

Long answer : There is a wide range of attacks that can be performed through WiFi. 
The impact of those attacks varies from knowing the pages you've visited to getting your password and\or gaining access over your computer.

Attacks like :

 1. [Man in the middle][1]
 
 2. [Session hijacking][2]  

 3. Exploiting a vulnerability in an application you have using [Metasploit][3]   

 4. Use your shared folders to infect your PC with a worm and\or virus. 

What you can do :

 1. Try to encrypt your traffic (use [HTTPS everywhere][4] )

 2. Use a good security solution like [Eset smart security][5] 

 3. Configure your WiFi Access point so it can reject non trusted connections (MAC filtering ?)



  [1]: http://en.wikipedia.org/wiki/Man-in-the-middle_attack
  [2]: http://en.wikipedia.org/wiki/Session_hijacking
  [3]: http://en.wikipedia.org/wiki/Metasploit_Project
  [4]: https://www.eff.org/https-everywhere
  [5]: http://www.eset.com