Hi Mark, firstly go into BIOS and check that your drives are set to auto detect and also check that you have the jumpers set correctly for master and slave.
Use a 98 boot disk and boot from A and at the A: prompt type FDISK.
This will allow you to check how youre original drive is set up and partitioned and also allow you to set an active partition on your second drive, although this isnt really necessary if you are going to instal XP on it as it should format as its installing.
After that reboot and select the drive letter of your second hard drive, double check to make sure that it is the correct one as you will erase any dat on it it, then type FORMAT X: where X is the letter of the drive that you want to format