Sharing Excel file

Help! I have 3 computers in a domain sharing an Excel document. They are all winxp pro. The file is on a workstation, and there is no problem from that computer. the problem is on the other 2 they can only open the file as read-only. the users have same rights. So what can i do to make the other two computers view the same file and modify. I know that i can view the file only on one computer at a time. I will apreciate any help

Thanks in advance..