How to wipe out OS and format a hard drive

hi guys =)
I ordered a new desktop without operating system (pre-format) cause I have a windows vista and windows xp cd. And i think it kinda messed up cause i put windows xp at first and I didn’t like it, then i put windows vista without deleting the previous one. Is there any way that i can delete both operating systems like delete everything until i see the black screen like a fresh one.? Like to wipe-out the OS. thanks guys

when computer is on insert the cd and reboot when asked if want to boot from cd hit any key. Well hit any key and it will start the install process again. Before you install the OS you format the drives and that will delete all previous installs then you can install fresh again.

Hope you can understand what I just said and what to do