If the system came from Dell with Windows, the license key is on the system board -- when you install the same version of Windows that shipped from the factory, it will automatically activate once you connect to the Internet.  You do not need a license key.  
Hello,    I recently used this laptop as a linux laptop even though it came with Windows. When I installed linux I did not save the windows product key. I now have windows again on my computer but it is not functioning well: it keeps crashing every few days. I can't currently find any viruses on my computer but I still want to reinstall windows due to this issue. When I try to install windows, it gives me the option to install by the cloud or by a local reinstall. So I guess I have two questions.  1) If I do the local reinstall, is that accessing the windows key on my bios or will I need to find the windows key regardless?  2) How can I find that windows key if I need to find it? I did accidently (I have had this computer for a year) pull off a sticker on the left side of my computer, I'm not sure if it was the windows sticker, because I can't find it on any physical location on my computer. 
Help with locating my windows key