First, let's relieve a little anxiety -- when you install XAMPP, a new MyQL installation will be created inside the XAMPP folder. So there will not be any over-writing of any existing MySQL installations. Thus, your data is safe. (You are backing up your MySQL data aren't you?) If you are not, start doing that.
Second, you don't need to worry at all about the original my SQL installation. As far as the Windows registry is concerned. When XAMPP is installed, it makes no Windows registry entries.
Now things get a little tricky.
On any one machine, you cannot have two MySQL installations running simultaneously on the same port.
This gets around to what you really want to use XAMPP for. If you want to use XAMPP as a Web server, AND use your existing database as a source of data for your Web applications, you can do that. You'll need to know a little bit about my SQL connections, but it is certainly doable. You simply will not use the XAMPP be installation of my SQL. (There is also the possibility that you may wish to move your data from the existing installation into the XAMPP MySQL installation, and that's relatively easy to do.)
If the two things (your database and XAMPP) have nothing to do with each other, and you wish to use something like say WordPress with XAMPP -- then you will need to stop the MySQL servers for your original installation, before you start the MySQL engine within XAMPP. This means you will not be able to run both of them as a 'service (where they automatically start when Windows fires up), but will rather have to rely on using start and stop commands either in the Microsoft service manager or through the use command line tools.
If you wish to use the XAMPP Apache only as a Web server, and not use any database in connection with it, then you will simply never start the XAMPP MySQL database.