Unmanaged DLL fails when called from web service (permissions ?)

[David Taylor]

I'm having what I think are permissions or security-related issues when
calling unmanaged code from a web service.

I have a VB ActiveX control from which I've generated a .NET wrapper using:
tlbimp.exe <VB OCX> /out:<NETWRAPPER DLL>

I've then created a new "Hello world" C# project, added the netwrapper DLL,
added it to the references folder and built everything. When I run/debug the
web service and invoke the "hello world" method, I find that I always get a
failure from the underlying OCX (the return code is a valid
application-specific error).

If I make a simple forms or console app, instantiate the object and call the
method in exactly the same manneras I do in the web service then everything
works fine with no error.

I'm not sure where I should be looking (first development with .NET & web
services) so any help is appreciated.