For many users this is piece of cake, but as I had to do the process recently, I decided to write a tutorial on it, just in case anyone has trouble.
First of all, insert the OEM OS cd that comes with the machine, reboot, and press F12 to go to the one time boot menu.
Now, you have the LEGACY and UEFI booting options. Although UEFI has some advantages (for what I have heard), the truth is it only worked for me using LEGACY, but you can try another option, so more information on the topic is found here.
Next step would be selecting an appropiate language and, more important KEYBOARD. You will be able to change the keyboard later, but it's good to have it set properly from the beginning.
In my case, I decided to install a totally new copy of Windows, instead of upgrading the previous. And created a big enough partition for that. I left some hard drive space unpartitioned, because I'll do it later with Linux.
And after that, it's pretty straightforward, in a few minutes W7 will be starting up correctly, except that you still won't have the proper drivers to use your laptop's full capabilities. What you have is only the OS, but no connectivity, no graphic options, and a number of huge other things missing.
So the next thing is to get the proper drivers, and for that you can go to the Dell site. In the Driver CAB Files you will find what you need.
You should download and unzip this, and then go to the Device Manager (Go to Control Panel, change to "Large Icons" view and there you find it, because in the "Category" view it's hidden), where you will see that most of the devices have been found but are not properly working, so you proceed to update the drivers of each one of them (networks, audio, video, everything), and finally, go to the Nvidia website and look for the driver NVS 3100M. Download it and isntall it and voila!