First off, we need to figure out the problem. This is not a replacement troubleshooter, rather this is a solution to a problem that VALVe has failed to acknowledge.

First off, try EVERYTHING they suggest in the troubleshooter. You can find it at https://support.steampowered.com/kb_article.php?ref=9402-MKEN-1517

If that doesn't work, there's a fix that they mentioned in my support ticket that seems to help with some issues.

The following has solved this problem for some Vista & Windows 7 users:

Exit Steam.

In Windows, click on the Start button > All Programs > Accessories

Launch the Command Prompt by right clicking on it and choosing "Run As Administrator"

Type in the following command and press enter:

net localgroup Administrators /add Local Service

Then, type in this command and press enter:

net localgroup Administrators NetworkService /ADD

Close the Command Prompt window and restart your computer.

Then, launch the game again.

If none of those work, then we've identified the problem as an issue with DirectX 10, 10.1, or 11.

Open your steam library and right-click on Team Fortress 2. Open the properties menu and click set launch options.

There are three possible launch commands that you can use, but only one will work for your computer depending on the version of DirectX you are using.

DirectX 9 or earlier: You shouldn't have this problem.

DirectX 10: -dxlevel 100

DirectX 10.1: -dxlevel 101

DirectX 11: -dxlevel 110

Close the properties menu and try it out. You'll notice that your screen resolution has changed. That's ok.

You'll also notice that particle and fluid effects run much smoother in DirectX 10 or DirectX 11 mode.

Disclaimer: I cannot be held legally responsible for any damage this solution may cause, however unlikely - it is up to you to chose whether or not to actually use it. However, if something does break, I will try my best to help you fix it.