Problem configuring linksys rounter (model RT31P2) to make a web server behind it visible

Hi,

I'm having a HARD time configuring a linksys router to make a web
server (IIS using port 80) that sits on my PC (behind the router)
accessible to the outside world.

Here's the network hardware bits:
subscribing to Cox communication's highspeed internet service, wiring
look like
cable -- cable modem -- linksys router -- pc
status: normal internet activity is just fine; web server is running
fine (internal testing is fine).
firmware verision 1.30.01 (upgrade via the firmware's feature failed).

Disabled all firewalls that I could find on the PC. Then, with the
firmware -- configuration,
In the Setup tab
----------------
(basic setup)
a) obtaining IP dynamically
b) router ip (local ip address): 192.168.5.1
c) network address settings (dhcp)
local dhcp server enabled
start ip address: 192.168.5.1 (????)
number of addresses: 110 (default 50)
dns 1: (I entered the primary dns from cox)
dns 2: (I entered the alternative dns from cox)
(advanced routing)
NAT enabled
dynamic routing disabled

Then, attempted the following:
1. enabled DMZ for 192.168.15.x (the PC's IP)
Outcome: did not work.
2. enabled port forwarding under the Application & Gaming tab,
application start end protocol enabled
webserver 80 80 both checked
Outcome: did not work
3. tried the above 1 and 2.
Outcome: did not work
4. tried the QoS (fine tuning?)
enabled http 80, still did not help.

Linksys technies tried to help but to no avail (part of the above is
their suggestion), what else could I try? What could be the
culprit?

Many thanks.