Installing XP Pro on Vista installed computer

I have a Dell Inspiron 530 with Windows Vista Premium installed. I'd like to install XP.

So, I have an installation CD with XP Pro SP2 and a key but when I insert the XP CD I am informed that:
"Installation is impossible because your computer has a newer version"

This is my first problem. The second problem is that I needed drivers for XP format. Where can I find them? How do I install them?

Use a Linux Live CD and format your hard drive. And regarding your drivers, in my opinion you can use your original CD which came along with your pc.