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That's not hard to do. You can open your current solution and add a new project. The project type should be "class library." In this project you put whatever classes you want. The classes and methods you want to be accessible should be marked public. The classes and methods you don't want other projects to access should be marked private or internal.
My suggestion is that you make a class with a name like XmlUtilities. This class should have a series of public static methods to parse XML.
In your other projects / web applications / batch jobs that need to call your XmlUtilties, you right click on the project name in the solution explorer and add a reference to your new class library.
Your class library will compile into a .dll. Your other projects, once you add a reference, will make a copy of the .dll and put it in their bin folder. So each project / website / etc. will have a copy of the .dll. Normally that would be fine. If you are really worried about having multiple copies of the .dll, you can copy the .dll into the GAC (Global Access Cache) and then have all your other projects add a reference to that copy. That's a much more involved effort, and it sounds like overkill for your project.
Good luck.