For just his files like word and stuff: Just delete the separate files For his programs: Go into control panel, click Add/Remove Programs and remove the selected programs To wipe the entire hard drive: Go to my computer, right click the hard drive, and do Format... BE sure you have recovery disks...
Okay, the only reason you would get that message is if you were actually trying to run the OS while erasing the drive. Yeah. That's not going to work.
Just boot from the recovery discs. The setup procedure will probably ask you at some point whether you want to erase all files. Just say yes. *shrug* That's probably the easiest method.
Be prepared to call in and activate Windows, as that's usually necessary on a re-install of OEM recovery discs. Also be prepared to spend an afternoon downloading Windows Updates and re-installing all your programs, etc.
If it comes with a "free trial" of anti-virus software, just uninstall it and download AVG Anti-virus & Anti-spyware. They work, they're easy to use, and they're 100% free. What a great combination.
Make sure you backup any data that you need off of it first. Also, unplug any other harddrives you have before doing it....some people who's names I won't mention have formated the wrong drive before. Wouldn't want that to happen to you.