The only reason I use DMZ is to make it easier on myself. I get tired of continued programs not running due to port problems and the time it takes to configure games is a pain when you cannot find what ports are needed.

For example tigerwoods 2007 took me forever due to EA not advising on what ports are used. This was a long time ago and I suppose I no longer require DMZ but then the problem we have no would be with both computers.

Fair enough. I don't play internet games so have never encountered those problems. My machines are all on standard NAT.

If it is a NAT/DMZ compatibility thing it's beyond me.

I used to have to set my ports for each game and I agree, it was a pain.

You meed to go into ADD REMOVE PROGRAMS
Select WINDOWS COMPONENTS
Select NETWORKING SERVICES
Click on the DETAILS button
Select uPnP interface (turn it on)
This helps your computer to automatically open ports for your games.

Also..
Are both computers using the same domain name for the network? The default for winxp is MSHOME, run the network setup on each again and make sure they are on the same name.

Other than that you can ask somone for remote assistance via msn messenger and if they know what they are doing they can check it out and fix it for you.
I would look into it for you if you want.. just page/mail me and we can set up a time for me to see what is going on.

Shouldn't the NAT be taking place on the router now, not on one of the PCs?