computers not listed in network places

i have win 2003 server and we have 140 computers in network 90% of computers are runing on win xp pro and 10% 2000 pro my problem is that when i open the my network place on my server i found that there are just 11 computers are listed tehn i refresh it then it goes 58 computers again i refresh it show 7 computer i can't see all computers over there so how can i solve this problem i have to make one schedule system but i m facing this problem so i can't create it. plz guide me .