Hello dear lady, by definition as a silver surfer you are a dear lady, just in case, to burn an iso image onto a cd is not the same as copy. Think of an iso image as a single box full of parts. By putting all those parts into one box it's very easy to carry them around but just opening it doesn't put them together into something usable. Burning an iso image to a cd or dvd is really very simple, the burner, IMGburn by the previous recommendation, will open the box, read the instructions and put it together into a complete program for you.
Starting your computer from that cd means to press a key at the very beginning of startup, right after you hit the power button, that allows you to select what the computer should start from. Typically you press either the f12 or f9 key above your numbers. select the dvd drive.
Be warned, Microsoft is ending support for XP very soon and since you say you are a surfer your computer will be much more susceptible to online virus attacks. You may want to consider buying a copy of windows7 or 8 and installing that now rather than later since you want to clean up your system but they might be a little heavy for your system to handle. If they are you could download a copy of Lubuntu which is free and you can try it out before installing by "burning" it to a DVD and starting the computer from it. If everything works and you like it the install will be a piece of cake. Lubuntu is a lightweight version of Ubuntu which should easily run on your computer, has no known viruses that attack it and will be very similar to what you are used to with XP although better organized if you ask me.
https://help.ubuntu.com/community/Lubuntu/GetLubuntu