Mystery problem:shares, permissions, AD?

I have one DC on a network (w2000 server)and two more servers running the same software that are member servers, one running exchange and the other runnning ISA. The problem is one I have not been able to pinpoint as of yet. The network seems to be running MUCH slower than it should and I noticed something odd when checking permissions on a user folder. The user name associated with the folder was not "resolved" when I brought up the permissions, there was instead numbers and letters. This was done on the (only) DC.(I have seen it on other related screens later also and will resolve in a second or two). I am quite sure this is also related to the apparent slowness of a rather large network application that runs on two of the w2000 server machines. I am questioning AD,DNS and DHCP.... What has me pulling hair is that it seems like the DC is unable to resolve what resides in its own system. I have no clear,or hazy for that matter, idea of what to do to start the process of getting things up to speed. I may have a DNS-DHCP setup error?? Can anyone give me an idea of "normal" DNS- forward, reverse lookup settings? I think I could remove the settings for the reverse lookup as I am a single domain. What are your thoughts?