If you have a *@*.edu email address you can get 7 pro for really cheap: http://www.microsoft.com/student/discounts/theultimatesteal-us/default.aspx
Some computer shops sell secondhand computers cheap, some still with XP.
Are you stupid? Why would you want to want to downgrade to an obsolete OS.
Lebump
Just get Windows 7 starter on cragslist