How Can I Wipe Hard Drive but keep OS for free?

I am planning on buying a new pc and want to sell the old one (this pc) But I don't want someone being able to look up credit card info, personal data, or passwords etc etc. I have heard that it's still possible even if you format the drive. But the thing is I want to keep the Operating System (Vista) on... show more I am planning on buying a new pc and want to sell the old one (this pc) But I don't want someone being able to look up credit card info, personal data, or passwords etc etc. I have heard that it's still possible even if you format the drive. But the thing is I want to keep the Operating System (Vista) on the pc because well.. I said before I want to sell it! Is there a way to completely erase the hard drive where nothing can be recovered and keep the OS? Is there a FREE solution?
Update: Wow that first answer was no help. I want to do more than just reformat and I want to somehow keep the OS on there since I don't have the vista disc
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