<<The DLL-file I created with VB 2005 could not be registered with Windows, so I could not call the function StartMe from within the EXE.>>
How are you creating the DLL I've been using the VB.NET command line compiler at work to build DLLs and then call them through EXE files, and haven't had an issue. (Though I don't believe this would work from VBA, at least not in Excel 2003).
If you know the function signature for the function you want to call, you can use Declare Sub or Declare Function to link to the function in the DLL. For example:
Declare Function MyFunc Lib "MyDLL.dll" Alias "MyFuncA" (ByVal Param1 As Integer, ByVal Param2 As String) As Boolean