Have you got any other CDs apart from the MS Office CD ?
Most new PC purchases DONT come with a "proper" Windows XP install CD (Microsoft stopped companies doing it because people were installing their Windows CD on other computers).
What you normally get is a "recovery" CD which is an special image of Windows that will only install on that PC.
Check to see if you have a "recovery" CD.
If not maybe you have a hidden partition on your hard disk that contains the Windows image.
Some companies, rather than supply a Windows recovery CD, put an image of Windows on a hidden partition of your hard disk and you can re-install from that.
Check the documentation that came with your PC to see if it tells you how to do a recovery of Windows.
If neither of these are available to you (the recovery CD or the hidden partition) then you MAY be able to get a recovery CD from the company that makes the PC.
Phone up their help line and see if they can supply you with a recovery CD (or maybe go into PC world).