The directory in which Steam related files are located can be different to mine, I have a partitioned hard drive. Meaning that I have two drives D:\ and C:\. So check where your Steam is installed before looking for the file.
Bad texturesIn-game you may encounter purple textures on the environment, or your weapon is really shiny like someone just buffed the hell out of it and more.
This means that the file that look safter this texture has 'broken'. I have found that there are two ways to fix this, either exiting Steam and then getting back into it, or verifying cache files.
Verifying cache files: to do this you go into to Steam and go to the 'My Games' tab, then right click the game that is having the problems and click 'properties'. Now open the 'local files' and click 'Verify integrety of game cache...'
Just let it do what it has to do then you should be right to go.
If it is still appearing, exit Steam and load it again. It should be right then.
All round fixesPeople have many problems with Steam, the most common fix for these problems is the clientregistry.blob file. It becomes corrupt very easily. It is located here; D:\Program Files\Valve\Steam.
Once deleted exit Steam and load it again, it will redownload the file again and you should be right.
Interface problemsIf you are having problems with your user interface (the thing you use to look at your Media files or the My Games list or the Store) delete the file SteamUI.dll located in D:\Program Files\Valve\Steam.
"I ran Steam and nothing is appearing, what do I do?"Since Steam is not apprearing there is only one place you can stop it, by press ctrl+alt+delete. Clicking on the "Processes" tab, click on the Steam area and click 'end process'. Click 'ok' to the next message that appears. For future reference dont end a process unless you are entirely sure what it does, as it make reck another program running.
Now delete the clientregistry.blob file and run Steam, if the same thing is happenning leave it to load, adventually it will. For me this problem has never persisted. There is also another file called 'Steam.dll' that could be deleted if neccessary, this is also located in the same directory as the files above.
Note: The effects of deleting the files will only take effect once you have re-run Steam, so as to allow for re-download.
"None of this stuff has fixed my problem, what should I do?"Search for the problem here: http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/home.php, and if you cant find the solution contact Steam Support. They take approximately 3-5 business days to respond.
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I hope this stuff helps