Hmmm...hmmmm..."newbie", huh? I'll start there.

The DNS addresses you use have to point to a valid DNS server. Having outdated addresses could account for slow login. The PCs might have accreted multiple entries over time as the configurations got changed around, and new settings were added and the old ones weren't deleted.

If your company's big enough to run your own DNS servers, then you shouuld point to the current official one. You mentioned branch office, which means it's more likely that the offices should use the DNS provided by their local ISP. In the lucky event that somebody remembered to keep the papers from the ISP with the DNS address, you're home free.

If not, as is usually the case, when you get right down to it, you really only need a working DNS, not any particular DNS. It's just a courtesy to use your own ISP's server. You could ping the addresses you find on those PCs to find a live one, then try it as DNS by doing a lookup for something like www.habitablezone.com. If it works, use it, delete the rest (or maybe you should write them down; it can be useful to know a few backup DNS addresses if you're having trouble with the usual one).

How many DNS addresses? Usually two is a good idea, preferably two located in different ISPs, for backup purposes. One is the minimum, zero you're flying blind on the Net. More than two doesn't buy you all that much, after suspenders and belt, what's left?

Final thing: If these are XP PCs, they may try to register themselves in a DNS, which eats up time, especially with some bad DNS addresses in there, and I can't for the life of me see the advantage. It may mean something on a big corporate LAN, but not home and the small office scenario you're probably looking at. I'm not logged in as an admin right now so I can't look at the Network settings dialog to refresh my memory exactly, but somehwere in there is a checkbox about registering your PC in DNS. Make sure it's unchecked.

Hope that helps.

(Rereading your message, I may have misinterpreted it, but I'm not sure. By "DNS IP", do you mean the IP address of an actual DNS server that looks up addresses for you; or do you mean the IP address used by the PC? The latter is an entirely different story than the one I wrote...lemme know.)