I need help! I have an Inspiron 1520 that has Vista installed on it. I am trying to upgrade to Windows 7. I find the software and I want to purchase, but I need to know will the product key still work since Microsoft does not support it anymore.
Windows 7 has reached end of life and Microsoft have not allowed sale of new Retail Keys since October 2013... Any keys purchased for Windows 7 are therefore going to be a bit suspect (and likely non-genuine).
That system is from 2007 and it should be able to run Windows 10 Version 2004. The cost of a full retail license is going to be higher than the overall cost of second hand hardware with a Windows 7, 8.x or 10 OEM License (which can all be used to install Windows 10). However you can install Windows 10 without a Product Key to run it unlicensed. When ran unlicensed there will be some restrictions when it comes to personalisation of settings and a water mark on the bottom right hand side of the Desktop but otherwise it should be usable and will continue to recieve security updates unlike Windows 7 and Windows Vista. Running Windows 10 unlicensed will at least allow you to assess its performance before purchasing a license. Windows 10 when ran unlicensed isn't as aggressive as previous Windows versions and for example will not forcibly log you out.
You cannot perform an Upgrade Install from Vista to 10 but can perform a Clean Install. Upgrade Installs were in any case problematic and usually resulted in impaired performance. You'll need to back all your files onto an external hard drive as anything on the system will be removed.
I provide pretty detailed instructions here on downloading and clean installing Windows 10:
Note your system is a Legacy System so make sure that you use the MBR Partition Scheme and the NTFS File System when making the Bootable USB with Rufus (you'll also need to use the older version of Rufus if making the USB on the Vista system and there is no need to bother splitting the install.wim file as the file format NTFS allows files which exceed 4 GB).
Windows 10 should have all the system drivers inbuilt for your model or obtain them via Windows update. There is no point installing Dell Command Update for a model as old as that.