First, read the documentation that came with the PC. It may tell you there how to re-install or restore Windows.
Did you get a restore CD?
Many PCs come with a restore CD. It is not an actual Windows CD, but will restore windows and other software that came on the origial hard disk.
As someone else has said, the restore image may be hidden on the hard disk.
As the PC boots up see if there is an option to do a restore. It may say something like "Press F12 to do a Restore".
The words may be slightly different, and it may only appear on the screen for a few seconds.
If none of this works you may have to buy a copy of Windows XP, You can get a OEM copy of XP Home for about 60 pounds.
Note if you do a re-install you ay lose EVERYTHING that is on the hard disk. EVERYTHING.