No it is not safe!
I suggest installing a software firewall before you disable the NAT device.
I have two machines networked behind a router with an additional software firewall installed on each.
Try Outpost Free
http://free.agnitum.com/
Absolutely painless to install and use. Auto detects AV and any networks you may be running. The only thing I had to sort out was that it wouldn't let my XP Home Desktop communicate with my Vista Home Notebook while allowing the vice versa. The fix was to make the network 'Trusted".