If you work with the Live Office Comptaibility enabled in Xcelsius, changes are that you cannot open an Excel file from the network anymore. However when strating Excel and then navigating to the file on the network in Excel itself, the file does open.
This can be fixed by de-selecting the Excel option: Ignore other applications.
Excel 2003: Go to Tools -> Options -> General -> de-select ignore other applications
Excel 2007: Go to Icon -> Excel Options -> Advanced -> General -> de-select ignore other IDE applications