Want to install Windows 7 in my Dell Inspiron all in one?

It will depend on the system model. If it has a Skylake CPU (6th generation) you'll need to produce a new Windows install media set -- none of the Microsoft media will work.  

If the system has a seventh-generation Kaby Lake CPU, there's no support for anything older than Windows 10.

For sixth-generation Skylake:

www.dell.com/.../how-to-install-windows-7-on-systems-with-skylake-chipset

For seventh-generation Kaby Lake:

www.dell.com/.../microsoft-windows-operating-system-support-for-intel-kaby-lake-processors

social.technet.microsoft.com/.../windows-7-support-for-intel-kaby-lake-and-skylake-cpus