I get this error: 'Steam.exe is not a valid Win32 Application'
This error can be the result from several causes:
You had a virus or trojan on your computer and removed it with an anti-virus tool, but the Steam executable was damaged.
If you were infected with a virus or trojan and your Steam installation was damaged, the best thing is for you to uninstall Steam and do a new clean install once you are certain you've removed the infection.
Your hard disk had a bad sector and some of the Steam.exe was lost or damaged.
If you are having problems on your hard disk, make sure you run scandisk in thorough mode, checking for bad sectors and defrag your disk once there are no further problems. If you are unable to run scandisk or it fails to complete, your hard disk might be having problems, please contact your hard disk manufacturer for further assistance.
Your internet connection or Operating system had some problem during a Steam update that damaged the Steam executable when it was being downloaded to your system. This might include running out of disk space.
In this case, you can try doing a repair of your Steam installation using the Wise installer tool in your Steam directory or from your O/S control panel add/remove programs option. When you select Steam, you are given the option to remove or repair the installation. Repair will replace the basic install files that might be damaged, and then Steam will update itself to the current version when you run it again.
If you are out of disk space, you will need to do disk cleanup (use the wizard in your operating system) or perhaps uninstall applications that you no longer need. Always be sure to run scandisk and defrag after removing applications from your computer.
You might have a bad download of the SteamInstaller.exe program.
In this case, uninstall Steam from the control panel add/remove option and then get a new copy of the latest installer (always get the latest installer from Steam only).