Do you have a floppy drive on this pc? If you do, download a Windows 98 bootdisk from bootdisk.com and create a boot floppy. You want a boot floppy with CD Rom support.
http://www.msu-hb.de/download/boot98se.exe
When you boot with the floppy, obviously choose "With CDROM Support" when the menu option appears.
Make note of which drive letter it assigns to your CD drive. When it gets to the A: prompt, (assuming your CD drive will be lettered "E:" for this example), type:
e: (tap enter key)
Now, type:
cd i386 (tap enter key)
Now, you will want the DOS-based XP setup file, which is NOT setup.exe. So, type:
winnt (tap enter key)
XP setup will now begin. If you want to start completely over, I would FDISK and then reformat the hard drive first before going into XP setup. If you just want to repair your current XP installation, do a Repair Install. Here's how:
http://www.personaltechpipeline.com/189500364