Can I change what happens if I long press my power button while it is turned on in my Dell Vostro 3590?

@The techie 279  No.  Long pressing a Power button always performs an immediate forced shutdown.  This is true for all PCs, not just Dell PCs.  The reason that function exists is so that you have a way to power off the system even if it is completely frozen and not responding properly to any input.  A long press is a last resort, and it's important to have this function available because otherwise it might not be possible to easily shut down a frozen system if you're using a laptop that has a battery, especially since modern laptops do not have easily removable batteries.  But that is not something you should be doing on a regular basis.  A long press on the Power button is NOT the same as clicking Shut Down in the OS.  Clicking Shut Down in the OS performs a "graceful" shutdown.  A long press on the Power button performs a hard shutdown, which is equivalent to suddenly disconnecting the power cable on a desktop PC that does not have a battery backup.  That should only be done when you have no other way to get the system to respond.