Is it possible to use iMac as a monitor with HDMI signal from a Windows PC?

Just a comment i don’t have space to add another monitor in addition to my older iMac. So using my older iMac for an extra monitor is helpful. I also need my older iMac for some of the programs that are no longer work on newer Apple computers.

iMacs are computers, they are not external displays. Please read Target Display Mode (Target Display Mode) and you may have a very old iMac that will work. However, Target Display Mode is not as reliable, easy to setup, and requires expensive cabling. Overall, you are much better off purchasing an external display.

Understood.

Best of luck then.

No.

Not possible unfortunately.