You shouldn't/wouldn't unless you are a Technet or MSDN subscriber.
Windows XP Home Edition, if it came with your computer or was
purchased later, should be on a CD (or - for some OEMs - on a
recovery partition of the hard disk drive.) You should have said
CD (or recovery partition) in your possession and likely should
have made a backup copy of it for safe keeping (as well as the
other pertinent information - like the product key.)
Did you lose yours (lose the CD, erase the recovery partition, not
make a backup copy, cannot read/no longer have the product key?)