The below steps may not delete all data. Some settings could still reside in the Administrator's registry account. The only sure way is to do a factory reset.
Create a new admin account with no password then login and delete all other user accounts in Control Panel.
Use TFC and CCleaner to delete any additional temp files.
Delete the Page File and disable System Restore.
Use CCleaner to wipe the free space of the hard drive with a 3 pass wipe. Erasing method is under options > settings. The dropdown box will give you the 3 pass option. Also check Wipe Data Streams, Cluster Tips, and MFT. Wiping disks is under the Tools option. Be sure to select Free Space and not Disk
The Dell allows a factory reset, if the recovery partition is intact, by pressing Ctrl + F11 at boot then releasing both keys. This would essentially wipe the drive and set it back to date of first purchase.
I am not sure if the Samsung allows a factory reset.
If you are up to it you could reinstall XP using the iso in this post. First link.