Removing Data off a Hard Disk

Hi

I’m responsible for a number of desktops and notebooks for my company and all store around 50,000 confidential records. I need an absolutely fool-proof software package that can completely destroy data on the hard disks – BUT not destroy the data still on the disk.

In other words I delete some files and need software to remove that deleted software from the disk to a level fit for the NSA. I don’t want to format the disk to remove it.

Price is a factor so if there are a variety of excellent packages it would help me to recommend a few.

Not that I need it but I want this data removed so not even the nsa could recover it. My job depends on it.

Thanks
B