Last bit first:
If you send a picture via email it will simply appear as an attachment to that email. The recipient won't see anything else that you see whilst viewing it.
To change the program that you open any file type in, go to the location of a relevant file. (e.g. open your 'My Pictures' folder, or wherever else you store your images). RIGHT-click on a random image. Select 'Open With'. Put a tick against 'Always open files of this type with this program' and select the relevant program. (NB: It's a very long time since I last used XP but I seem to recall that you won't initially see that 'Always open . . . ' box. If you don't see it, instead of directly selecting a program, click on the option which lets you browse through programs instead. The box should then appear).
Having got all of that out of the way, I'll point out that I regard it as pure madness to use either of the Microsoft image-viewing programs that you've referred to. Irfanview is about a million billion zillion trillion squared times better than either of them!
http://www.irfanview.com/