Hi R44
Am no expert but this should sort your problem. First of all make sure you have the original windows cd's that came with the computer.
You have to go into the BIOS and configure the pc so that it will boot from the D: drive first ( or whatever letter is allocated to your cd rom drive).
To do this shut down the pc and reboot...... during boot-up you will get an option to go into set-up ( you will see a note on the bottom or top of your screen telling you which F key to use..... it,s usually F1 or F2.
When in the set-up page scroll down till you see the boot sequence. You will then be able to configure it so that it will boot first from your cd rom drive.
There are a couple of ways to reformat the C: drive.
Go to" start" and double click MY COMPUTER. rt click on the C:drive. From the drop down list select FORMAT. For XP the normal file system is NTFS so select this option.
Shut down pc and insert your windows disk......restart and hopefully all you will have to do is follow the instructions from there on.
Incidentally if you ever want the dos C: prompt go to "Programmes" then "Accessories" and you will find it half way down the list.
Hope this helps. You may get some more expert advice than mine so watch the post for a while.
Good luck
Willby