Vista - Dll

Hello all. Here's my problem. I want to create a "one instance dll". I
have a .NET program and an Excel file. I want them to talk together by
this dll. My current problem is that Everytime Excel start, it creates
a new instance of my class. What I need, is everytime my .NET
application start, it register a delegate in the dll function. So when
the Excel file needs, it call a function in the DLL that trigger the
event to all the register delegate from the .NET programs that are
running. I don't know if i'm clear...

Someone can help me ? I think what I wan't is kind of a dllserver...