I agree with oldbob, there's got to be something very wrong with a repair shop that replaces a hard drive, but doesn't install the operating system.
They will need to configure the machine to boot from CD (by going into the CMOS setup program), boot with the XP CD (or recovery CD if they have one), and follow the instructions.
After XP is installed, they will need to install all the motherboard drivers (they should have a separate CD for this, unless the PC is one of those brands that comes with a recovery CD rather than an XP CD). Then they will need to install any additional hardware drivers.
XP will offer them the chance to activate online, but before they can do that they will need to get their internet connection working. As OB says, they can choose to activate by phone - in any case they have 30 days in which to do it.
Once the machine is activated they will need to download and install all the Windows updates that have been issued since their CD was manufactured.