Your reply crossed with my editing of my reply earlier.
What I did was set the port forwarding up on the Router having set a static IP address on the XP computer and the DNS numbers from the ipconfig /all command.
I then set the default IP address and DNS on the DVR to match
I was able to login to the DVR from the XP machine and found an iMac compatible DVR access program which was a surprise as most of my gear is Apple.
As I pen this I can see the cameras on the screen here.
Thanks again
ps, now that I am on the iMac I don't see how the static IP address idea helps as the DNS numbers are given by the ipconfig /all command