Just back from the library--dependable computers! And spent time at Microsoft Support. type in the search box, "How to Reinstall Windows XP" It's a 4 part series, each section about 16 pages long (illustrations, too), with lots of "make sure you do this before...." and read the entire thing. For example, an installation discs will not do the job of a start up disc, and then there's the no more support sections, and, how not to follow the directions for service pack 2 if you have service pack 3--which you should have installed....And "How to turn on and off System Restore" is well worth reading. Especially if it does or does not have service pack 2 or 3.
Initially makes your head want to explode. Seriously. But, like rebuilding your first engine, it's a follow the directions, and get ready to spend time. Lots of time. Do not scan, but read Part I, especially about what you should have at hand to do this. That part is very well written for the DIYer..
And you do have the program discs to install the system, right? Glad HP made me download XP's 9 discs before I could operate this computer. I'll shop for a hard drive and more RAM this weekend.
I strongly suggest you have another computer next to the one you're working on so you can access Microsoft's Support. Yeah, I've never done it, so, I'm a newbie, but, it looks like a walk in a park with a few thorns along the way.
I hope I'm wrong on that.