Every SERIOUS VB6 application, ended up requiring
that you import some functionality from the windows API or another DLL.
However if you were looking for a .NET equivalent and
google did not take you to the right page,
there is a document (a little old I might say) called
Microsoft Win32 to Microsoft .NET Framework API Map

I remember a couple of other pages that were even more complete,
but right know I can only remember http://pinvoke.net that gives some info about APIs