Update (2026): Fedora 18 and KDE 4 are long obsolete. On KDE Plasma 5/6 use System Settings > Workspace > General Behavior > Clicking files or folders, and the config now lives in ~/.config/kdeglobals.

This solution works for Fedora 18 (KDE). If you use another version or distro, give it a try anyway.

Open a terminal and edit the file:

vi ~/.kde/share/config/kdeglobals

Add (below anything that exists) or modify:

[KDE]
SingleClick=false

Log out and log in again and you're all set. You will now need a double-click to navigate into a folder or open an executable file.

Thanks to Peterius: LinuxQuestions thread