You could start with C# 4.0 / .NET 4.0 which is supported on XP.
.NET 3.5 has a much higher install-penetration than .NET 4.0 though, so if you're really concerned with making installations hassle-free, you might want to consider starting with C# 3.0 / .NET 3.5 (using Visual Studio 2010).
Also, I would suggest starting with Windows Forms (WinForms) instead of WPF because it's super simple and easy to wrap your head around. You can get something built very quickly with WinForms. In WPF you'll be spending at least a month studying before you start to feel confident.