When the Steam client cannot start or crashes, please try the following:
- Navigate with Windows Explorer into the installation directory of Steam (eg C:\Program Files\Steam\) on your hard disk.
- Delete all directory files, except for "steamapps" (this contains your downloaded games).
- Delete all files except the file "Steam.exe".
- Start the Steam.exe file. All needed files are loaded automatically and you can log in with your existing Steam account.
Now try to make sure everything works as desired.