Wiping drive clean in a RAID1 envoironment

I am trying to prepare an old Dimension 8300 for disposal. It is running Win XP with RAID1 and I am unclear about how to run a disk cleaning program in this environment. Do I need to break the RAID relationship to make sure I clean both drives or will that happen automatically when I run the disk cleaner? This computer contains lots of personal financial information so I need to be certain I am getting everything off the drives.