Deleting all personal files

[cheddarcaveman]

I won a Dell XPS L502x computer in a competition last month but have found that I prefer my iMac and MacBook Air and am therefore going to sell this one. I now need to delete the few files etc that I have put on there. What is the best and safest way to do this while retaining the Windows 7 system installed? I do not have a Windows 7 disc but I did make a Recovery disc as I initially set it up.
Help would be appreciated. Anybody want to buy it by the way?
It is here Dell XPS 15 L502x i7-2630QM Backlit Keyboard 1GB nVidia GT525M 750GB 6GB | eBay

or I can sell privately.

 

[Kaktussoft]

Why not install again using the recovery disk? Also read this http://www.sevenforums.com/tutorials/276046-cipher-command-overwrite-deleted-data-windows.html

 

[Britton30]

You can install CCleaner (free version), delete your personal data then fun CCleaner with a 7 pass overwrite. also run the registry cleaner function.