ethel - if connecting via the same router .... wired or wirelessly ..... that's a network - both machines are part of the same subnet segment and so do share network resources (even if not files).
hubble
run the wire - wireless is slow .... and if you are intending to dl music/video and viruses....Wireless error correction overlays ethernet so on sustained data transfers ... the lag can be huge.
both PCs should be running firewalls (and virus/adware scanners) as a matter of course.
the router's DHCP server will provide some degree of protection from casual hackers - and if you look in the router's setup there could well be a hardware firewall already at work.
as a final "defense" make sure
both machines are password protected,
neither machine has a user account that appears on the other
and that the machines have different workgroup names