I also have a DLL for using the registry that was written for VB but that could work, couldn't it???
Not if it was written IN VB, unless it was created with VBAdvance or similar tool that allows a vb dll to have exportable functions. And you'll need the VB runtime dll as well.
If you are using a decls file that uses standard windows API, then you don't need to worry. It's possible that some api calls could be WinXP/2000 specific, and you'll need to test that. But usually, with api calls, just compile and forget about them. With the api registry calls this should be the case.
If the decls file references a custom dll, then you need to include the dll with your program and it would probably be best to just install it into the same directory as your program. Though you could optionally place it in the system32 directory of windows. Good practice nowadays though is to place it with your program to avoid version conflicts that can arise.