You can do a factory restore, which would be the easiest.  There is a recovery partition located on your computer from which you can reinstall the OS to factory condition. If you want to install from a DVD, you need to order the disk from Dell that you opted out of when you ordered.   Alternatively, you can download an installation ISO from the link below that matches your product  key/license.  Going this route though, you must call Microsoft to activate Windows.
I have studio 15 bought around 15 months back. Had Windows 7 premium along with it. But hadn't  taken a cd for the same. Want to re-install the OS now, so how can i do it without the CD...???
Reinstalling Windows Os without CD