Microsoft has published an amazingly in-depth guide for designing user interfaces when using Windows Forms (.NET).  It is geared towards their recently released OS, Windows Vista, but the principles apply to applications running on XP as well.  You can browse the contents online or download it as a PDF.  I have a printed copy of it at my desk at work and occasionally thumb through it just to peruse.  It is extremely helpful, especially if you are unsure whether or not to use a particular control to perform certain actions.  Here's the link: Windows Vista User Experience Guidelines