Find on amazon, newegg, or ebay.....when you get the CD, do this steps:
1- Do a backup of all your documents and drivers (with mydrivers you can back up all drivers of the system)
2- Reboot the pc and press F8, or the key that show you the boot menu.
3- Start from the XP CD that you bought previously.
4- Whe the installation load all drivers from the beginning, press enter to install XP. The install program ask you if you want to do a recovery from another installation from XP or do a new installation, select a new installation (Esc).
5- Th installation program show you the HDD and the size of this. Delete all partitions and create the new partitions
6- After this,you select the partition for the installation files (usually the first partition) and the files system start to be copy to the HDD.
7- The rest of the process is guided for the installation program.