SortedList values ....?

One thing you could do is to create a second SortedList. The (unique) key for this list would be the key for the first list prepended by the year and the value would be the same as the first list. This second list would then sort by year and then in the same order as before.

The code would look something like this:

using System;
using System.Collections;

class Program
{

     static void Main()
     {
         SortedList sl = new SortedList();        
         // arbitrary values used
         sl.Add("1/1/2006I11833802002",10);
         sl.Add("1/1/2007C12473522004",20);
         sl.Add("1/1/2007I12473342002",30);
         sl.Add("1/1/2007I12473352002",40);
         sl.Add("1/1/2007I12473362002",50);
         sl.Add("1/1/2007I12473382002",60);
         sl.Add("1/1/2007I12473442004",70);
         sl.Add("1/1/2007I12473452004",80);
         sl.Add("1/1/2007I12473532004",90);
         sl.Add("1/10/2006C11867322001",100);
         sl.Add("1/10/2006C11869032003",110);

         SortedList sl2 = new SortedList();
         string key = null;

         foreach(string s in sl.Keys)
         {
            key = s.Substring(s.Length - 4) + s;
            sl2.Add(key, sl[s]);           
         }
       
         int year1 = int.Parse(sl2.GetKey(0).ToString().Substring(0,4));
         int year2 = 0;
         double total = double.Parse(sl2.GetByIndex(0).ToString());
         double taxRate = 0.1; // or whatever
         for(int i = 1; i < sl2.Keys.Count; i++)
         {
            year2 = int.Parse(sl2.GetKey(i).ToString().Substring(0,4));
            if (year2 == year1)
            {
               total += double.Parse(sl2.GetByIndex(i).ToString());
               if (i == sl2.Keys.Count - 1)
               {
                 total *= (1 + taxRate);
                 Console.WriteLine("{0}   {1:N2} It's Total += Item.Tax for this {0} yr", year1, total);
               }
            }
            else
            {
               total *= (1 + taxRate);
               Console.WriteLine("{0}   {1:N2} It's Total += Item.Tax for this {0} yr", year1, total);
               year1 = year2;              
               total = double.Parse(sl2.GetByIndex(i).ToString());
            }
         }
         Console.ReadLine();
     }
}