Although the .NET runtime platform can be installed on Win9x, along with some testing tools, I believe that the VS.NET development tools can only be installed on an NT-based OS. I know for a fact that the release version will not install on 98. I haven't tried on ME, but I'd assume that's the same deal.
If you can get your hands on just the .NET Framework, which was free at one time and might still be, you might be able to use the command-line compilers and write your code in notepad, a'la Java. Also, there's an article somewhere on Builder about getting VC++ 6 to compile C# projects, so that's an option too.
I do believe that NT4 is supported, assuming it's been service-packed to the gills, so if you happen to have NT4 you're in business. If not, it's probably pretty cheap. Good luck.