Word opens document as read-only

We have Win2K computers running Office XP. Intermittently, users close a document and save it to the server, when they double-click to open it again, it opens as read-only. The only work-around we have found so far is to either have the user rightclick on the document and choose open or open Word first, then open the document. Anyone know why this happens and/or any fixes? Thanks in advance for any help!