1. Open Steam's Library and select Team Fortress 2
2. Right click it and then select Properties.
3. Click "Set up Launch..." and then type in: "-dxlevel 81" (without quotes).
4. Click "OK" and then start the game, close the game and go back to step 3, but instead of writing, you will erase the "-dxlevel 81".
5. Click "OK" again and then start the game again, configure your video settings to the lower level and set it on the lowest resolution (640x480).
6. Play the game normally, and you don't need to make all these steps each time you start the game, just one time is enough to set the Direct X level forever (if you want it back to normal state type "-dxlevel 91" on the launch options).