You can't downgrade from within Windows so you will need to boot from CD. One question though why in the world would you want to DOWNGRADE to Windows XP it is a dead OS?
windoes XP stopped to be the most used OS during 2011 so you can't realy call it a dead OS and if you own a cheap lap top (the kind you can buy at wallmart) you'll want to install XP over 7. Using the low contrast UI still pretty demanding for a cheap build.
Windows XP is a dead OS in the sense that Microsoft is ending all support in 2014 including security updates. It's crazy to have 7 and downgrade to XP when you are just going to have to upgrade again unless you like being part of a botnet.
And direct x newest version don't work in XP so some games don't work. Its a myth that Windows XP is a ton more efficient. Windows 7 does fine AND the system was designed for it. I would be completely shocked if you could even find XP drivers for a new consumer laptop at this.point as.manufacturers have no reason to make them. XP should be avoided if at all possible.