Most commercial computer vendors these days don't give you an operating system Cd/DVD when you buy the system (but it can usually be purchased for an addition fee). The vendors usually create a hidden partition on the HDD, which can be accessed from a boot menu to reinstall the OS if necessary, and it's usually an image file that contains the OS and device drivers for the system as it was supplied from the factory.
Since your HDD crashed, you might try and contact the vendor and see if they'll sell you the Cd/DVD for your system, although, since it's Win XP Home, then it's most likely an older computer and out of warranty, but you can still ask. You might also ask if they can sell you a newer version of the OS (Vista or Win 7, but they may not. Be sure and ask about compatibility as well). Otherwise, your best bet would be to purchase another version of the OS. Here's a link for Newegg that shows the various versions of MS OS Home Edition software:
Be aware that if you purchase an OEM/System Builders version, then that version is only valid for the life of the motherboard that it was installed on, and you would need to purchase yet another version if you ever decided to upgrade (you don't specify whether this is desktop or laptop computer). Microsoft's license for OEM/System Buiders version is only valid for the computer that it was first installed on.
Here's another link for the FULL version of all the current MS operating systems: