This one was bugging us for a long time, until we got message from Jay (thanks!) who had similar problem and was willing to help.
Story 2 -- Mysterious Component
The problem itself is mysterious. From time to time it happens that Visual Studio stops loading any add-ins. ReSharper is installed, but its menus are suddenly all gray and it is not listed in the "Tools \ Add-in manager" dialog. It happens after installing various products, it could be service pack, SQL server tools, Visual Studio components - anything! Some users install them and all works fine. Some users loose their favorite productivity tool.
Logging, debugging, trying to reproduce for may be several years(!) led to no result. ReSharper was simply not loaded into the process, though all registration information appears to be in the right place. We checked encodings, verified files are not corrupted, verified registry access rights - all we can think of. As you can guess now, the reason was not anywhere near.
After lots of email exchanges with Jay, including mini-dumps, Process Monitor traces, registry excerpts, configuration files and such we suddenly found ourselves staring at the right thing. It was msxml6.dll. Actually, it was the fact that there were no msxml6.dll in the call stack. Instead, there was msxml3.dll.
We use .addin files to register our add-in in Visual Studio. It is XML file describing add-in, containing information about primary assembly, descriptions, load options and Visual Studio version compatibility information. The latter was that msxml3 was not able to process and thus Visual Studio refused add-in as non-compatible. In fact, msxml6 was there in System32, but it somehow happened to be not registered as COM object. Using regsvr32 on msxml6.dll repaired the system, resurrected ReSharper and enabled customers to enjoy our productivity add-in again.