Visual Studio's strangest error message yet....

All I want to do is add a file to my project.... but "the computer says no"

I get a message box that says:

"Attempted to read or write protected memory. This is often an indication that other memory is corrupt"

So what does this mean? Well, googling for the error reveals all sorts of hits - e.g. this thread is quite interesting http://social.msdn.microsoft.com/Forums/en-US/csharpgeneral/thread/6adca20b-649f-41a4-8fa1-09534882d76c/ - but I haven't found a solution (yet)

Ho hum!


Update - I think this was something to do with the fact that one of my projects was a 2.0 target - while everything else was 3.5 - but really I'm still in the dark.