Admins often blame the ISA firewall for a variety of network problems. I have to admit that I sometime fall into the trap. We all get lazy and forget that you have to troubleshoot network problems beginning with layer 1 and working your way up.
So, before you even think about checking into problems with the ISA firewall, you need to make sure the NICs on your problematic clients are configured correctly. Here's a good article in the MS KBthat can help you with that:
Thomas W Shinder, M.D., MCSE
Sr. Consultant / Technical Writer