You can't download XP officially from anywhere. (and any unofficial places it can be downloaded from are very high risk and there is a good chance you'll get a pre-installed virus if you download one from unofficial places)
Contact your computer manufacturer, or look for one on ebay with a license and the original CD.
Many PCs have the recovery disc built in ie 'recovery partition' (HP for one),its accessed by pressing one of the 'F' keys on start up. Worth googling your PC to find out...