Networking Problem?

I am getting a strange error on my windows xp machines. I have a windows 2000 server that is the domain controller, DNS, and DHCP. The network is small 10 users 2 printers 1 network attached storage. When i open up task manager all the user process show “unknown”, and when I go to properties of my computer and go to view the computer name I get an access denied error. It cannot verify the domain which shows “unknown” and it is unable to read domain membership, access denied again. All users are able to access the shares on the network. But this problem seems to affect authentication at times. Does anyone have a suggestion? I don’t want this to get worse.