VS 2010 and Office 2007.
Full message:
could not load file or assembly 'office, version=12.0.0.0, culture=neutral, publickeytoken=71e9bce111e9429c

Started getting the above message on my VB.Net app that uses Excel on a particular XP machine. It worked fine on other XP / Windows 7 machines.
The XP machine in question had the right version of Excel loaded and had worked previously with my app.
After lots of searching I eventually found out that it was missing all the assemblies from c:\windows\assembly with a publickeytoken=71e9bce111e942.
There is a tool that you can use to reinstall them, but what I did (be it right or wrong) was to just copy the whole c:\windows\assembly from a working machine to the non working one (you can't copy individual entries) and that fixed it.
What is odd is that old versions of my app work fine and it uses Excel alot, but the new version didn't work until I copied all the assemblies over.
Any way, hopefully this will be useful to someone.

Kristian