DllImport on framework 1.1, in IIS 6 (Windows Vista)

Greetings,
I've already posted this message on aspnet.security group (think its related
to security issues), but since didn't get any response, I post it here.

--

I have a problem, and been around it for some days without finding a
solution.

I have an aspnet 1.1 application that works perfectly on Windows XP, Windows
2000 and Windows 2003.

When trying on Windows Vista (set application for aspnet 1.1 on IIS), the
application crashes Internet Explorer with the message:
"IIS Worker Process stopped working and was closed"

Doing some debug, I found that the errors is caused when accessing a method
from an external dll (win32).
I use dllimport to define the method, something like:

[DllImport("name.dll", CharSet=CharSet.Ansi, EntryPoint="functionname")]
public static extern string functionname (string SomeParameter);


I've searched a lot for a solution, and tried different aproaches, like
giving full access to dll file to everyone, impersonating, enabeling
unmanaged code debug, moving dll to system32 path, checking that the
application is with Full trust level on IIS, etc.

I know the problem is not the dll path, since aspnet is finding it (if I
remove the dll from bin folder, it throws an error of file not found).


I thought that it could be a problem with the dll on vista, but if I create
a winform application, the dll works fine.



Appreciate any help on this subject,

Nelson R.