Yes. C# is the language of choice when it comes to .NET development. Since you've programmed in Java, you'll find that the language looks very familiar (it looks a bit cleaner though and has some very nice features like delegates and you can even use pointers if you really want to, although you won't need to in most circumstances).
It's SWF (System Windows Forms) or MWF (Managed Windows Forms - a lightweight version of SWF written in managed Code using System.Drawing - it frees you from the WinAPI that's underlying classical SWF) all the way.
There's also GTK-Sharp, a GTK+ binding for C-Sharp, but since you're developing for Windows, users will find it strange to install a third-party GUI framework for your application.
I wouldn't recommend using WPF though. It makes direct calls to the underlying graphics hardware, which means it's quite fast, but not portable at all. It will only run on x86 systems (not SPARC, etc.) with DirectX compliant hardware rendering units (graphics cards) and it will not run on a system with strong hardware abstraction (like most Unices like BSD or Linux or Solaris).
Novell also stated that they will never implement WPF in the near future due to it's strong hardware dependencies (Novell wants Mono to be portable across many processor platforms). Don't use WPF. Software is no longer just a throw-away-product and when it comes to operating systems we're in the age of spin. The platform you're running today isn't necessarily the platform you're gonna run tomorrow. WPF is a proprietary and patent-encumbered technology developed by Microsoft and Intel. If you use it, you'll find yourself in a very nice Win32-on-x86 vendor lock in. And this means that you'll most definitely find yourself completely re-writing your application sooner or later. Just write portable software from the beginning, even if you don't need that portability today. I'm telling you from experience, you will need portability tomorrow. Definitely!