The easiest way: Just hook each hard drive up, use fdisk to remove all the partitions, and then recreate them. Then do an unconditional format "Format /U" that ensures that most of the data is being erased. If you're paranoid, there are utilities out there that will completely erase all data on the drive, just in case the previous owner liked child porn, etc. Norton Utilities' SpeedDisk will do this, BCWipe, and a few others, I could send you.
When the drives are wiped and you re-create the partitions, just pop in the Windows install disk and install.