From Penn's security quiz:

Which of these is NOT a reliable method to help secure your computer?
Installing a firewall
Using complex, strong passwords
Keeping your virus protection and operating system patches up to date
* Only file sharing with people you know and trust

Correct!
What is often misunderstood by users of free peer-to-peer filesharing applications is that many of these services have weak or nonexistent security and privacy controls. They often expose their users to adware, spyware, and other nuisance applications that hoard a computer’s processing power and can prove extremely difficult to remove. Even the basic filesharing built into your operating system can be tricky to set up. If you do not clearly understand how to set file and directory permissions for the material you want to share, or how to manage the users to whom you will be granting access to your system, you may want to turn off filesharing altogether to protect your computer against compromise.