First check to make sure that Steam.exe is
not set to
Read-Only. To find out right-click on Steam and select properties and make sure Read-Only is unticked.
If you have AVG Anti-virus (all versions) steam will fail to install/update. With the recent updates AVG now sees Steam as a Virus and will stop the installation/update, the only way around is to un-install.
If you have any of these
programs running you will need to close them down in the task manager.
Finally go to your steam directory and delete the file
Clientregistry.blob and then start Steam back up.