At first you need the freeware tool "ArtMoney" which can be found here: http://www.artmoney.ru/e_download_se.htm
Hard way: (WILL WORK FOR ANY VERSION OF HALO 2)
Start ArtMoney (or a similar tool). In ArtMoney you select "Halo 2" from the "Select Process" drop down menu. Then you go to the control-settings of Halo 2 and change the mouse-aiming speed to the standard setting (3). Now alt-tab and change to ArtMoney where you click the "Search" button. Here you search for an "Unknown" value of "Float (STANDARD)". This will take some minutes as it searches for all values in the memory. If it is finished alt-tab back to Halo 2 and change the mouse setting to "4". Now alt-tab back to ArtMoney and click "Sieve" and choose "was increased" and press OK. After it has finished sieving, go back to Halo and set the speed to "1". Go back to ArtMoney and press "Sieve" and choose "was decreased" and press OK. Go back to Halo and set it to maybe "5". Go back to ArtMoney and "sieve" again, this time choose "was increased". You will notice that the amount of values it finds drops from the thousands to hundreds and so on. Do it until you have maybe 30 values (around 50 sievings). Now you click on the green arrow to "add all" values from the sieve to the right window. Now it should be easy to find the two values for you (they are 230.00 and 115.00 when you select the speed of "10" in the Halo 2 menu). Remember to now SAVE the table (= all values from the right window) or you will have to do this again. If you are sure which values change your mouse-speed you can also delete the others. If you found your values do as described in the example below.
Easy way: (THIS WILL ONLY WORK IF YOU HAVE A WORKING TABLE FILE
Start ArtMoney( should work with similar tools as well), then Halo 2 in window mode (to better switch between the game and ArtMoney later). In ArtMoney you select "Halo 2" from the "Select Process" drop down menu. Then you load your pre-made table ("Halo2.amt" for example). After this you can see two values.Change the two values to maybe "990.00" for the first and "400.00" for the second value (the first alters horizontal, the second vertical aim-speed) and your aim flies.. The value for horizontal aiming needs to be higher to make it feel equally fast since the game handles h/v differently. Experiment with the values to find a setting to your liking. It doesn't matter what settings you have in the game-menu. You can also quit ArtMoney and the value in the memory will stay changed until you use the game menu again or restart Halo 2.
IMPORTANT TO REMEMBER: If you start ArtMoney and then Halo 2 REMEMBER to select "Halo 2" from the list of processes in ArtMoney, otherwise when you load up the table with the two values, it will say "<NO VALUE> where something like "230.00" should be.
IMPORTANT TO REMEMBER #2: This might be considered cheating if you play online so be fair and tell others about this if you use it.
Have fun!

dex
EDIT: My tables will not work for you, those are different values for each system, but a premade table from YOUR system will work and you can use the "easy way" if you created one. Everyone needs to search for the values first (second example). This method will work for any version of Halo 2.