Question : ISA Blocking subnet internet access

Hi, please can some kind soul help me?
I have 3 subnets all happily talking to each other through a router.  An ISA 2006 server is connected to one subnet and acts as the gateway to the internet.
PCs that are on the same subnet as the ISA can all access the internet (NAT proxy through the ISA).
However, the other two subnets appear blocked and can't access the internet.  In fact the 'blocked' subnets can't even ping the ISA server.
In the attached diagram PC A can access the internet, but no others.

Attachments:

ISA-Prob.bmp (216 KB) (File Type Details) 

ISA Setup

Answer : ISA Blocking subnet internet access

It was SPOOFING!
If I switch spoofing off KB838114 (Create DWORD: HKLM/SYSTEM/CurrentControlSet/Services/FwEng/Parameters/DisableSpoofDetection=1)
Everything pings and I can browse the internet.
I've even undone all the previous and everything still works!!