Domain

Hi everybody ,
for last 2 to 3 days i m facing strange problem, i
m having windows 2000 server SP4 with around 100 clients (win xp ), the problem is,i m not able to add any new machine to domain. it keeps giving me errors eg. comp name already exists on domain, or u don’t have rights.i have updated my server but still no use, also tried for viruses but nothing seems to working. PLSssss help me !!!!!!!!!!!

Thanks in advance