Steam Crash: CMultiFieldBlob(pSerialized): Bad field - extend past end of blob

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=216

To fix this problem you need to exit from Steam (if it's running) and delete a file in your Steam directory called:

ClientRegistry.blob

This would typically be in:

c:\Valve\Steam

or

c:\program files\Steam

The next time you start Steam it will re-create this file, do a brief update and then you should be able to play.

Re-installing Steam on top of an old install may not fix a problem like this, you would have to use the add/remove program in the control panel.