I think I am posting twice but C/C++ for my favorite. As for C#, it is actively being pushed by microsoft for the programming language of choice for Longhorn, with all that managed code stuff. As I understand it C# is kind of like a wrapper for the C language, however nothing wrong with that. With the way windows works, the majority of your code can easily and should be standardized. Unfortunately there are still control freaks like me out there that want to know how everything works. However even if the programming language that microsoft is pushing for Longhorn is C#, the OS was still written in C.
As for creating your own programming language you can do that from any language starting out. For instance, C++'s first compiler simply converted the code to C and then used a C compiler to make it an executable. You will have to get into file management and string manipulation as well as come up with an outline for the intent and purpose of your language.