How do I use a DLL with C#?

I am trying to use a DLL created with Visual Studio 6.0 from my C# application created with Visual Studio.NET 2003. As soon as I go to add a reference I get "A reference could not be added...Not a valid assembly or COM component..."

If you can help me, please reply.

Thanks,
Jim Mason

1. register it w/ com
2. wrap it in C++ w/ .net
3. use system.runtime.interopservices to do a DllImport for each function you need.

ex: Playsound

[DllImport("winmm.dll")]
public static extern bool PlaySound(string sound, IntPtr module, int soundFlags);

Thanks MadHatter. I'll give it a shot. I have a lot to learn here.
Thanks Again,
Jim