Have this one to do this week on several Home land security servers.
To be frank there is nothing available that will clean all the data on a HDD omitting the O/S and leave it in a state were data cannot be recovered, i.e. wiping all information but leaving you with a good operating system.
As it stand to day even freeware data recovery utilites are quite capable of recovering information from a HDD, even though it has been formated.
There are however several FREEWARE 'Disk killers' that will, by bypassing the operating system and going in through the BIOS wipe, or ZERO write the drive complete so that nothing remains to be recovered except null values for every data sector, a very VERY secure way of cleaning a drive, but then requires a re-install of the O/S.
So if you really don't want anything recovered off the computer you bought or are worried it might contain something you really do need to remove the only sure way is to zero the drive, Microsoft have been active in this field for a while now and have offered several options on the following site:
http://www.microsoft.com/athome/secu...ecomputer.mspx
For just cleaning down to a secure state any file or folder try downloading a copy of WIPECLEAN by Avantrix, around £10 to download, allows you to make a fairly secure wipe of any file or folder and the recycle bin, but will not wipe files already deleted or not on the file recovery listing in the recycle bin. i.e if you can't see it it can't wipe it.