No and Depends are the answers to your questions.
Long story short, you can't always install XP on newer computers because there aren't any XP drivers for the hardware. So, before doing much of anything, you will probably want to make sure there are XP drivers for your system. If there aren't, you're just out of luck and will have to either use Vista or look into something like Linux.
But installing Windows is not that difficult overall. Put the disc in the drive, reboot the system, follow the on screen instructions. If you have a SATA drive, there's an extra step where you have to load a SATA driver, but it's not that difficult overall.
I recently purchased a new computer that only came equipped with Windows Vista. I bit the bullet and decided that I could except such a buggy OS for the fancy new features my computer has. Now, I routinely get 5-10 Blue Screens of Death per day. Combine that with the fact that many of my programs consistantly crash and the OS doesn't boot correctly 25% of the time, and I am done. I wish to get XP on this system. So I have 2 questions:
1) I have an old "Gateway XP Installation" disk from my old computer. My current computer is an HP. Is it possible to download this on my computer?
2) If I do just have to purchase XP, how dificult is it to install on and take off Vista? Is there a site that walks me through it? Or is it rather simple? Please help!