closing files

We are having a problem with files not being closed on our server. Often when someone closes a file on their machine (windows 98, NT 4.0, 2000 w/latest service packs) it remains “in use” on the server. Recently we found some still open that hadn’tbeen opened in a month. We’re running NT server 4.0 sp6a on the file server. The open files are primarily autocad drawing files, but it happens also with word 2000 and on occaision excel 2000. Any suggestions on how to remedy this would be greatly appreciated.