firstly try to delete data by shift+delete or from recycle bin. If still data is not deleted then,The simplest way to do this is to format all the drives but that too may not delete all the data and there are always chances to recover them.
I can suggest you 1 tool that will remove all ur data permanently forever and no other tool can also be able to recover it.
Advanced System Optimizer has a tool named Secure Delete that does the permanent deletion of the data
but once the data removed can not be recovered anyhow.
u can download this from cnethttp://download.cnet.com/Advanced-System-Optimizer/3000-2094_4-10147659.html?tag=mncol