Steam Crash: The MSS DLL is incorrectly installed in the Windows System directory

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=263

If you get this error message then for some reason you have mss32.dll file either in your c:\windows\system32 folder or somewhere in the system path.

You need to delete that file from your system.

If you are unable to find it in the system32 folder, then use your windows search function, be sure to enable searching in hidden and system folders in the advanced options.