The second step is actually a three step process. The images below will help show just what to look for once you have the registry editor opened up. First however you start off by going to the Start>Run command line and typing "regedit" followed by pressing the enter/return key.
The first image shows what that prompt along with the regedit window will look like plus how the registry is organized very similar to a directory with it's own sub folders in each branch or often referred to as "hives".
The second image is a back away view of the first where you have already opened the "HKEY_LOCAL_MACHINE" branch. The first highlighted the SOFTWARE then microsoft hive as you can see from that after scrolling down to it. The second here shows one more scroll once that is opened down to the Media Player listing or main key.
You'll note that once the main key is highlight you go upto the menu bar to select "delete" from the drop down list seen in "edit". Remember you only want that one key and nothing else. Despite using the add/remove option that key will still be left there from WMP 11 and simply needs to be removed followed by a normal install of 10.
When initially trying out 11 on XP I ran into not being able to get 10 to install once 11 was removed in the add/remove. Upon seeing that I went right into the registry having spent enough time in there and saw that one key still lingering and removed it to see 10 go on like a piece of cake.
Once deleted simply restart the system to see the registry change made permanent and you shouldn't have any problem seeing 10 go right on with ease. Before rebooting however you'll want to back out of the registry by closing each hive up especially if you have the remember last folder viewed option checked off in the folders options.
That will prevent returning to the exact place if you should need to open the registry again. Just like the directories you see listed in Windows Explorer there's a plus sign next to each item you can open or close up.