I confirm what
@harlan4096 says, Windows 7 is no longer supported, although a patch exists to maintain until 2023.
Even if you have an excellent Antivirus (which is Norton), security flaws are exploited every day by hackers on vulnerable systems.
An Antivirus can't counter everything.
I could have recommended you Ubuntu which is a Linux system, but you need to know a little bit about Linux because on an Optiplex , few Windows systems can be installed because of the age of the computer.