Whats the process to downgrade Windows 7 to Windows XP?

I just upgrade my pc from windows XP to windows 7. But after that my PC does not start up & even in safe mode. I changed the boot to start from cd drive & then from hard disk & not responding and the pc run for hours with window start. Now I can't even downgrade to XP anymore as it not running from cd. So How can I downgrade to Windows XP from Windows 7?
