Well neither are technically "Microsoft tools", but I've used two different options:

http://json.codeplex.com/[/url] had a built up C# library to (open source) you can invoke from .NET code to reserialize XML as JSON.

I've also had occasion to use the ALTOVA serializer. It's very nice in interactive session, but does require licensing one or more components from ALTOVA, so this wouldn't be free, but it did yield performance enhancements to the plain code widget when we ramped up on the transform tool.