Saving excel workbooks and spreadsheets

We have a few WinXP/OfficeXP machines that have encountered a rather annoying problem. When hitting the ctrl+s keys to save a spreadsheet or workbook, a message box appears and asks whether I want to overwrite the file or not. (This occurs with brand new documents with unique filenames.) I've seen it only occur when saving to a shared network drive. All permissions are set correctly on the server. Any ideas on how to get rid of this message? Thanks in advance.