Cannot open blob archive file: CMultiFieldBlob(mem-mapped file): Failed to open file

http://support.steampowered.com/cgi-bin/steampowered.cfg/php/enduser/std_adp.php?p_faqid=240

Normally, if you get this error when you try to launch Steam, you can fix it by deleting the ClientRegistry.blob file.

However, if you also get this error when you try to connect to a server, and once you exit Steam you again get the above problem (Steam not opening at all) then you have a much more serious problem.

To verify this, when you try to connect to a server, no error dialogs pop up but if you look in your game console (typically ~) it says:

"SteamIDGetEncryptedUserIDTicket() failed with error1: cannot open blob archive file: CMultiFieldBlob(mem-mapped file): Failed to open file"

These specific symptoms indicate a serious problem with your operating system, most likely from some software that you installed. The only guaranteed solution requires you to reformat and do a complete clean install of the operating system and a complete clean install of Steam.

Using any portion of your old files might very well cause this problem to happen again.

The best way to format and re-install is to take your computer off the internet so you won't be vulnerable.

Assuming you have Windows XP, you can also download the full Service Pack 2 from Microsoft that you can burn on a CD, this way you can get all the latest security updates and O/S patches as soon as you install the operating system, before you have to put your computer online again.