This is just the case I still wonder now what the cause really is. By the fact that there is 5 clients and 1 server in the network behind a router. All of clients are running Windows XP professional and the server runs Windows Server 2003 for file backing up and file server.
The problem is only one-pair of them has a problem. The computer x1 rarely sees the x3 one and sometimes x3 is accessible but with very sluggish speed--almost 2 minutes waiting just to browse the file in explorer. Nonetheless x1 is accessible on x3 perfectly.
You may want to know that any other pairs don't have this problem. Even though x1 and server or with x5 or x3 with anyone. ping is good, time is less than 3 ms with anyone except the problem pair.
To alleviate the random problem like this, I really have no idea much. So, I have tried to change the file sharing to be more advanced and add the user 'everyone' and 'administrator' into the share permission. I forget to let you know that every single computer on the network uses 'administrator' as a login and they have the same password but the x3 one which normally use another user name as a login and administrator doesn't have any password. However, I have changed the password of administrator to be the same as others already. For you guys who wonder about firewall on x3, it's installed Norton Firewall and I did turn off to prevent a problem but frankly the problem was not gone.
But, then, it still has the same problem. I really frustrate with the windows network for the first time, not because it's working or not working, but because the symptom happens randomly so I couldn't figure out why it really is. When I go fixing, it seems works just fine. Nonetheless I use the alternative--set the x3 as a fixed IP--to solve this and it seems help.
After that, I haven't heard they have a problem again which could mean two things: the problem was solved or they don't believe me fixing that again :-/ ha ha ha if anyone passing by has any idea or else, please let me know. Thanks!