I haven't ran Portal for a while and my recording software can only record something like a tab. How do you make Portal not fullscreen?

Portal runs in windowed mode by default if you're using Source Unpack.

Options / Videos /

By tab, do you mean window? If so, you can just go to options>video>display mode>run in window. Or you can use the Source Unpack version of Portal, then it is by default in windowed mode but you can't see the borders.

you need to add the following to your startup cmd: -window -noborder