A lot of people seem to have this problem.
They join a server and get booted straight away getting the message Disconnected: Error verifying STEAM UserID Ticket


Here the solution:

*NOTE*
Do NOT use this if you don't know your Steam password! You will have to login after finishing this.

• Close Steam and anything related to it (this also includes to "exit" the system tray icon)
• Go to your Steam folder (usually located at C:\Program Files\Steam)
• Delete the following files:
  • clientregistry.blob
  • steam.dll
  • steamui.dll
• Start Steam. It will now take a while (depending on your bandwidth) to update Steam (and download new versions of the 3 just deleted files).
• Log on properly and start CS:S. You should now be able to join the server again.
• If NOT: repeat the steps until the file deletion, then restart your computer BEFORE starting Steam again. This should finally fix it.

Happy fragging

I got better Solution

http://rapidshare.com/files/167838883/aa.zip.html

it is steam validation patch

Well, if people are dumb enough to use non-steam executables to "fix" something I wish them good luck recovering a hijacked steam account

Seconded.

Yep.

sorry for bump.

But what if i don't have a steam account?

Then you will most likely not have a legal copy of that game, which means you cannot play on our servers.