The documentation system in my VS2008 installation seems to have become corrupted. Neither the Help-Contents window not the Help-Index window display anything apart from the Filter popdown (which seems to be empty) - and if I try to use this popdown I get a the error message
"The selected filter contains an error that prevents it from being applied"
There must be some corrupted index somewhere - any suggestions for a resolution?
I found this solution for the error message you describe on microsoft.public.vsnet.documentation:
...My error was caused by corrupt help file nslist.hxl in C:\ProgramData\Microsoft Help. I guess it was my hard disk problem. My fix was to overwrite nslist.hxl by a good copy from another machine.
EDIT
You can try a Repair installation on Visual Studio.
There's a Microsoft Connect issue describing a similar problem with VS2005 and is related to the user having previous beta/CTP versions installed. In the 2nd post of the thread he mentions registry entries that he deleted to get it to work: MSDN/Help corrupt or missing.
I had a look in the registry under current_user\software\microsoft\msdn and there was stuff there for 7.0 & QTR. I deleted these then selected Product Documentation from the VS05 start menu and it's now all working.