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

Long answer : There id wide range of attacks that can be preformed 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 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