Probably the easiest way, is to get a Linux LiveCD. Boot from that, then use the disk partitioner to remove the Mac OS X partitions and create new NTFS partitions. Then you reboot with the XP CD and install from there.
The Linux LiveCD is necessary because in my experience the Windows installer gets confused when it encounters partition types other than FAT12/16/32 and NTFS, and it will stall before you're able to even get started installing the OS. It's also good to have something like a Linux LiveCD around in case your system becomes overrun with malware and won't boot, but you want to try and salvage some data.
Probably any Linux LiveCD will do, but Knoppix and Ubuntu are the ones that kind of come to mind. Knoppix is designed primarily as a LiveCD, while Ubuntu is primarily a HDD based distribution, but it also lets you sort of try before you buy with a LiveCD option. Or at least it did, it's been a long time since I've looked at Ubuntu, but I doubt they'd remove that, since their install program was basically to clone the LiveCD and lay it down on the HDD.
I have been given a dell computer which I want to delete the BS operating system, format/wipe clean the drive and re-load xp. How do i do this?