Ok, what you want to to do is actually very simple. Start up your computer with your WindowsXP cd in the drive. It will boot from that.
It will start doing some things on its own, but you will soon be asked what to do. From there it will ask you where to install, and you will see all your partitions. At this point I believe you hit "D" and it will delete the partition you have selected. Do this for all of them.
When all you have left is Unpartitioned Space, press "C" to Create. Then choose the size. If you are going to have a multi partition setup again you can either set those additional ones up now or wait til you are in Windows. After you have created a partition, you tell windows to install to that one. Then proceed as normal.