You need to make sure your BIOS is set to boot from the floppy disk first. I don't know the settings for your particular BIOS, so you will have to figure that one out on your own. Once you do that, simply insert #1 disk, and follow the instructions.

Then make sure you have your XP CD ready. YOu can either repair XP, install over the top of the current installation, or format and start over completely.