Either:
You have a bootable floppy disk, USB memory stick or CDrom in the drive, and it does not contain an operating system (windows)
Or
Your hard drive is broken, has lost some (possibly all) information on it including Windows.
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First, remove all CDs, DVDs, floppies, USB disks etc and try again.
If it still says "not found" take it to a local computer shop (not PC world) and get them to give you a quick opinion on the state of the hard disk. You can also boot from the Windows XP setuip disk and make as if you want to install XP. XP will examine the disks in the machine and report on any hard disks it finds
If it says "no hard disks are available in this computer" or finds a hard disk but it is entirely denoted to be "unpartitioned space" then your hard drive has likely suffered a failure of such a degree that you should run some dos-based recovery software to recover your files, then buy a new drive and start again.
If the hard disk is reported as something like:
C: NTFS xxxxxxMB free space
Then the hard disk still has partition information but is likely missing some vital files. Reinstall XP on the drive. If it offers you the opption to re-install the operating system files but keep all your settings, then it has found enough of your data to proceed from.. otherwise you'll have to reinstall as if from afresh and reinstall all your programs. Your data (photos, documents) will be retained UNLESS YOU CHOOSE TO FORMAT the disk.
Investigate the root cause of the missing files:
If your kids deleted them, no problem. If the hard disk is old and dying, back up your data, buy a new disk and start over (again)