Lost HP Operating System Disk

A friend gave me an HP Compaq dc7600 but she could not find the XP Professional CD. She had attempted to delete her data and some special programs and had it in a mess. Drivers were missing on just about everything. I was able to download drivers and get the puter up and running but would like to have the operating system in case I need it. She had the OEM number and the Key numbers written down but nothing else.
Help would be appreciated.