Having 2 DHCPs on same Network

is the second IP in your exclusion range a typo? It says 172.20.12.15, that would mean you're excluding everything. you meant 10.15 right?

assuming I'm right your proper setup should be like so:

1. Add an exclusion range on your NT server that equals what you want the Netgear to give out.
2. Assign that as the only available range on the netgear.

No matter what you should not get overlapping IP's this way.

The problem is that if you have two DHCP servers handing out IPs the workstations are going to get an IP from whichever one responds first.  So while the netgear may hand out 10.10 thru 10.15 it may never pass out these IPs if the NT server responds to the DHCP request first.  Aramirezomni is correct that you don't want the two zones overlapping unless you want one of them to be a backup DHCP server.  

What are you trying to accomplish by having the Netgear hand out IPs?

I have consultants with wireless adapters who come in now and then. i want the wireless router to hand them a certain range ip addresses (172.20.10.10 to 172.20.10.15). This range in particular because i have created a rule on the ISA that allows it to browse the internet.  

the rang 172.20.10.1 to 172.20.10.15 is excluded. 172.20.10.10 to 172.20.10.15 is the range the netgear is supposed to allocate. But no joy

If you hard wire into the netgear box are you getting network?

Right now it sounds like a problem on the Netgear.  Without having the configuration of the netgear it's hard to say what the issue is.  What are the details of the wireless configuration on the Netgear?

The netgear has a section where u specify the ip address, subnet, gateway and dns servers. I have the following
ip address 172.20.10.9  mask 255.255.0.0 gateway 172.20.10.15 (ISA server),  DNSs 217.a.b.c, 217.a.b.d
There is a checkbox for using it as a DHCP and i have ticked that. the range i specified is 172.20.10.10 to 172.20.10.15  
Still a machine with a wireless adapter is given any ip address such as 172.20.10.73.