Unable to find Steam.dll entry point InternalSteamStartEngine

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=261

This is typically caused by a bad Steam install or something got corrupted on your existing Steam installation.

The easiest fix for this problem is to delete this file (usual location):

c:\program files\valve\steam\clientregistry.blob

If that doesn't fix this problem then your best bet is to uninstall Steam using the add/remove in the control panel, get this Steam installer:

http://www.steampowered.com/index.php?area=getsteamnow