i personally suggest you give windows 7 some more time. it will definitely sink into you and you'll love it like xp and vista.
you may want to look at this link Windows lifecycle fact sheet - Microsoft Windows
it basically says that Windows XP has reached its end-of-sale stage, this basically means that Microsoft is no longer producing and selling copies of Windows XP. any remaining copies of Windows XP can be bought in stores, however Microsoft's official support for XP will only be until April 2014. Also Microsoft suggest that you switch from XP to Windows 7.
Windows Vista is due to stop manufacturing and selling by October 2011. As with XP, any remaining copies of Vista can still be bought.
Below is the downgrade option as said in Microsoft's site. Not sure if theres a quick option to jump from Windows 7 down to Windows XP.
I suggest you check with your computer's manufacturer or retail store. They should be able to provide you with the correct procedure for you to purchase the additional license that you will require for Windows XP or Vista.