You will not be able to downgrade to Windows XP from within Windows 7.
You will need to boot from Windows XP disc, format all partitions and install a fresh copy of Windows. Two things that you might like to consider before:
1. Data loss, make sure you backup all your data, the process will erase anything and everything.
2. Also check if your computer and hardware has drivers for Windows XP. Some computers, especially OEM computers do not have drivers for Windows XP which will cause some problems.