Download and use DBAN (Darik's Boot And Nuke) to wipe it clean, then reinstall Windows. Be sure to get the drivers you'll need first. As an alternative, you could just restore to factory settings. The procedure for that should be in your laptop's user guide. It will format your hard drive then reload your software to "out of the box" status.
I'm trying to sell and alienware m11x laptop on Craigslist but i need to completely wipe the hard drive and make it "brand new" again. If anyone could help me i'd greatly appreciate it.