Hi just want to add a personal solution that might help when installing xp over windows 7. I had a notebook with limited resources and I installed windows 7 and turned out this was not good. Tried to install xp again but no way in hell was this going to happen with windows 7 installed. Not to keen on partitions and the sort I could not find a solutions, I decided to install Lubuntu (light weight Ubuntu for the not so powerful computers) from a usb which is simply to do and with a lot of luck Lubuntu offers to remove windows 7 as an option!!! Of course I went for it and it removed windows 7 really smooth. Afterwords I was able to install xp in a breeze and all was well.
Note Note Note Of course you loose all data and files of windows 7 so take care and back up if you want to save anything!!!