Sounds like enough of Windows got installed that Bios thinks it can boot off of your hard disk, but not enough was installed that Windows actually boots. It's likely that the BIOS simply has the hard disk set as the first boot device, and the CD-ROM drive somewhere after that. This will cause the BIOS to automatically boot off of the hard disk if it thinks it can. What you need to do is set the BIOS to boot from your CD-ROM drive so that you may load a new XP install disc, and reformat the hard drive and install again.
Depending on your exact BIOS (and they vary greatly), the process of getting it to boot off of a CD can be quite different. Basically, when you first turn the computer on, you will probably notice some text flashing across the screen, and somewhere in that whole mix, usually a screen will show up with a picture of either the brand of the computer manufacturer or motherboard. Look in the bottom corners of this screen (quickly, it wont be up there for long), and see what options are available to you. If you're lucky, your BIOS might have a "Boot Menu" option. This is usually F12. Alternatively, if it does not, you will have to enter BIOS setup by pressing either F1 or F2 during that BIOS splash screen.
Either way, once you've figured out what your options are, you'll want to press the corresponding key (F1, F2, or F12 depending on what options you have) DURING the BIOS splash screen to enter that option. The boot menu is fairly straightforward, simply select from a list of boot devices which you would like to boot from (in this case the disc drive). If it you have to enter setup however, things might be slightly more complicated. All BIOSs are different so I can't give you exact directions, but basically, you're looking for a place to specify the "Boot Order" or "Boot Device Priority." Once you've changed the boot order so that your CD-ROM is selected first, simply restart your computer with the XP install disc in the drive, and wait for the XP Setup utility to come up. Depending on your BIOS, your computer may still prompt you to "Press any key to boot from CD......" This will give you about 5 seconds to press any key in order to boot from the disc.
Hopefully this fixes your issue. If I misinterpereted the issue you were having, let me know and I will try to help you further.