There is a problem in win2000 with the DNS client service - it is known to occassionally fail to work, preventing name lookups working at all.
I found this at the office. On just one of a number of PCs, I've had to disable this service. It makes name lookups a lot slower presumably because windoze has to go to the ISP's nameserver but at least it works. Previously, about 50% of name lookups would totally fail and the only way to get it to work was by typing in
ipconfig /flushdns
ipconfig /renew
I never got to the bottom of it. Reinstalling windows might do it but I've got sp4 and loads of security updates there...