You just need to update your windows 7 including optional update and then connect your laptop with internet & insert windows 8 dvd/usb media then simply upgrade and then allow to download the installer. it may download 500kb-2mb file & then simply foll the instruction. you also need to transfer your personal file & setup at the stage.then you will be able to install windows 8. thank you  
I was having trouble to install Windows 8 in my XPS L401X because I didn't have the BIOS update for my laptop model which is necessary for installing Windows 8 and unfortunately Dell was not supplying the necessary BIOS update to me . But fortunately Microsoft supplied me with the update and now the problem is fixed and I have the all new Windows 8 installed in my laptop without any problem and it is running great. Al you have to do is connect to the internet while installing Windows 8 in your laptop. A small file will be downloaded first and will be installed then you are good to go ahead and install the OS.   Happy Windows 8 Installing guys :)
windows 8 installation fixed in xps l401x