Why would you expect an operating system to be present if you´re wiping the HD, which point is to zero it out?
And even if it remained due to failure of the steps given, how would opening an OS command prompt facilitate wiping the HD from the OS you want to wipe out? Only the Roadrunner can erase himself.
Easier to just boot the XP CD using the one-time BIOS Boot Menu key given on first screen, delete all partitions and create new one, full format NTFS. Wiping the HD is overkill unless you have data you need overwritten with zeroes so it can´t be pieced together by forensic experts. It provides absolutely no installation benefit unless there is boot sector corruption which otherwise blocks the install, and then the simple "Clean" command is sufficient.