I want to install Windows 7 in Dell Inspiron all in one, i changed secured to legacy in boot options, but not working.
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:
For seventh-generation Kaby Lake: