I'm not familiar with Centura Builder, so I am not sure at what level of abstraction your prior Windows programming experience is. If you choose the native code route using C/C++ and Win32, be sure to buy a copy of Programming Windows by Charles Petzold. Reading the first few chapters of this book can help you decide if you want to go the native code route as well.
If you choose the .NET Framework and C#, and if you do not need the version 3.5-specific features (such as LINQ for database access), I recommend choosing version 3.0. It is a built-in component on Windows Vista, so you don't have to package the runtime with your program installer unless you have to support Windows XP (which you probably do, so never mind :). With version 3.0, you get Windows Presentation Foundation (to use instead of or along with Windows Forms), which gives you a lot of graphical capability without a lot of effort. I'm not knowledgeable about CAD, so I don't know if WPF would provide the drawing functions that you need, but it may provide a great platform upon which you can write your own drawing routines.
EDIT: I missed your Windows version requirements on first reading, particularly the Windows 2000 requirement. I guess you'd have to go with .NET Framework version 2.0 and Windows Forms. I have to ask, though: Wouldn't it be cost-effective to improve your engineers' productivity by upgrading their eight-year-old machines to something newer and faster and therefore get rid of your Windows 2000 support requirement?