After installing Resharper 4.1 on Windows 2003 server, I got this message when I start Visual Studio 2008:
The Add-in 'JetBrains ReSharper 4.1' failed to load or caused an exception.
Error Message: The system cannot find the file specified.
Error Number: 80070002
I emailed JetBrains support and got a quick response. It turns out my system did not have Extensibility.dll installed in the GAC (Global Assembly Cache), and ReSharper requires this dll to run.
To figure this out, I opened explorer to the path "%windir%\assembly", which brings up a shell extension that displays the contents of your GAC.
The fix was easy, I just had to find the dll, which is installed with Visual Studio. Once I knew the location, I opened the Visual Studio command prompt and ran
gacutil /i "C:\Program Files\Common Files\Microsoft Shared\MSENV\PublicAssemblies"
Hopefully this saves someone else some time.