Seems to be caused by a corrupted Visual C++ installation (multiple versions can be installed side-by-side).
You can try to use the sxstrace tool to see if you can find which files are missing or corrupted.

• http://www.vistax64.com/software/245768-side-side-configuration-incorrect.html

(This question came up in a KB article discussion.)

I also found a related bug report (which includes an attachment of the error message) in case it helps:

• Bug 716291 - Latest update has effectively killed firefox on my laptop, something about side by side config being incorrect

I have this same problem.After getting a virus,I started in safe mode and downloaded avg after doing a system restore.Everything works great again except firefox,which I love.I have tried downloading the visual c++ (or whatever) ,it did no good.I have windows 7 and my laptop is a sony vaio...if that makes a diffrence...if anyone can walk me through how to fix this step by step ,and in the simplest terms possible(there are quite a few forums on this subject if you google it,however if you do not know computers you don't have a chance of understanding them) I would greatly appreciate it...:)