Team fortress 2 validating steam cache
If uninstalling is not an option, temporarily disable or exit the software before starting playing.
Download ZIP, and extract the for cool tips and tricks. Make sure you back up your binds and other custom settings before you do this.
Lately Team Fortress 2 started to cause problems with Garry's Mod.
- Copy over all the folders that apply to your game. Team Fortress 2 will say Team Fortress 2 somewhere in the folder name, Serious Sam 3 will say Serious Sam 3 somewhere in the folder name, etc.
You don't need to worry about any of the other game files, unless you're planning to transfer more than one game or something.
If you have Team Fortress 2 installed, you should complete steps above for it ( easy link ), and any other installed Source Engine game as well.