Downgrading the operating system of Compaq Mini CQ10-500SE

I need some help to downgrade my operating system from windows 7 professional to windows XP service pack 3. The problem is, every time I run the setup of windows XP, the previous OS can’t be seen that’s why I can’t delete to free some memory of my hard disk. I’m using a Compaq Mini CQ10-500SE. Need your help
