Win2000 DNS conflict?

Basically, I have three DNS numbers on my Win2000 client PC on a corporate network, the middle one being the Domain Controller DNS value. If I move this value to first or third, IE6 won't recognise a single URL.

I frequently get "page not found", so reverse the first and third DNS values, and more often than not this sorts it out...until I try to load the next page.

Primary Domain Controller runs W2K Advanced Server, going out through a Cisco Pix firewall and router.

Any ideas as to why I keep having to reverse the figures?