Time required: Two minutes.
Prerequisites:
1. Your account name and password. (deleting "steam\config" wipes your stored account credentials)
2. Access to the email your steam account is tied to. (you'll need the SteamGuard auth code)
Instructions:
1. Close steam (Right click steam icon in system tray > exit)
2. Navigate to your steam folder (usually "C:\Program Files (x86)\Steam")
3. Delete or move the folder called "config"
4. Start Steam.
5. Log on to your account.
6. Play.
If this does not work then try the following solution posted by Fug.
Here is how the client can fix this issue with relation to all source mods:
Delete all app/file/folder in steam EXCEPT the following:
+Steam.exe
+Steamapps Folder
+[Optional to delete] Userdata folder (which contain your screenshot)
Most people will tell to just delete the client_registry.blob or w/e it is. <--That used to work but now the above ^method is now more full-proof.
Hope this helps...
P.s. I think Valve are aware of this.