A while back, I posted a set of steps that can be used to help resolve package load failures in Visual Studio 2005. These steps are only designed to resolve known issues we've seen on systems that previously had beta versions of Visual Studio 2005 installed.
However, there are many possible causes of package load failures, and they can occur on systems that never had beta versions of VS 2005 installed. Many of the errors I've seen are caused by bugs in the packages themselves and not by bugs in Visual Studio. For example, this issue was caused by the package not correctly handling the scenario where VS was installed to a non-default path and had nothing to do with beta versions of VS 2005.
I recently came across a link to an article on the Visual Studio Extensibility team blog that describes some techniques that are useful for debugging package load failures:
This article is primarily aimed at developers who are creating their own Visual Studio packages, but the tips listed there are also useful for end users who are receiving Package Load Failure errors when launching or using the Visual Studio IDE.
If you are encountering package load failures and other workarounds listed on the MSDN Forums and blogs haven't been helpful, it might help to try the debugging techniques described in this post.