Build it as anonymous object in dotnet and then serialize it as Json, Asp.Net has the serializer built in.
string json = context.Response.Write(Json(new {
sEcho = 1,
iTotalRecords = "57",
iTotalDisplayRecords = "57",
aaData = new List<List<String>>
{ new List<String>{ "Gecko", "Firefox 1.0", "Win 98+ / OSX.2+", "1.7", "A" }}
}, JsonRequestBehavior.AllowGet));
Update:
Modified based on comments.