Step 1: erase the hard disk with DBAN (free download)
Step 2: install windows 7 from the installation disk
Step 3: install drivers and utility programs that Dell offers for this machine for Windows 7
I'd add a step 0: make a clone of the drive so you can go back to Windows 8 later. But that's up to you.
However, I wonder why you didn't simply a Dell 15z with Windows 7. It's the only one they offer at http://www.dell.com/us/business/p/xps-15z/pd So before doing all of this I'd contact Dell to see if they can sell you recovery disks for Windows 7. Or maybe you're still inside a 30-day return period? That would be the easiest.
I just bought a Dell 15z 5523 Ultrabook (i7-3517U Processor, 8GB DDR 3 1600 Mhz RAM, 256 GB SSD, Nvidia GeForce GT 630M 2GB card etc.) pre-installed with Windows 8, not PRO.
I have been using it for a couple of weeks and I just hate it. I know I know, some of us don't want to learn new things or be forced to change our ways just because of some corporate's whim....
That being said, I have searched quite a bit online, and still haven't found a reliable resource which can help me remove Windows 8 completely and install Windows 7 Home Basic. Have received a lot of contradictory advise.
Dell did not give me a Windows 8 installation disk. Just a drivers CD. I have the old Windows 7 Home Basic installation disk and drivers. Also, I have already activated 8.
If someone asks me to stick with Windows 8 or give benefits of 8 over 7 or tell me to install stuff like Classic Shell it would be a total waste...
Can someone give me a step by step guide or a link which has everything explained?