Go into the bios of your motherboard and set boot priority on the settings to cd rom as the first one. Rite now it mite be floppy or hard drive. Once you do that then the install should work, once installed go to the bios and change the boot drive priority back to hard drive for faster start ups.
When you first started the PC, you should have entered the BIOS (by hitting the Delete key) and set up the current date and time and set your boot order with your CD drive being first. You can change that later. If you've done that already, insert the Windows XP CD into the drive and restart the PC. Windows setup should boot up and take you through the format, partition and XP installation process.
If you want, you can tweak your BIOS after Windows is installed. Right now, you just want to get everything set up without getting fancy.
if you know everything is connected properly, and you are still getting the disk boot failure message and XP CD will not boot, then boot from a windows 98 startup floppy disk (u can download the files easily if you do not have one), use FDISK to delete all partitions, then create 1 primary dos partition, exit FDISK, type format C:, when it finishes, reboot from the WinXP CD and it should load properly