Invoking HTML Help

I've asked this question before and it was answered satisfactorally but it was so long ago that I can't remember the answer (and I can't find the thread searching either).

Basically, I have a VB project which I have associated a CHM help file and normally, this is enough to enable the user to bring up help by pressing F1. However, for some reason, in this particular project, it does absolutely nothing.

Can anyone tell me how to either fix this problem or invoke the help file manually?

Cheers.

1. Set the help file name in project properties.

2. Set the helpcontextid of each control to the id number of its topic file.