One of my clients has this issue. Windows XP PC access a Windows 2003 SBS server. After a Windows update in June the time to open an Excel file on the server takes 30 sec to 2 min. Other PC’s that have not updated work fine. After searching online it looks like MS has introduced a File Validation add-on. This is the cause of the slowness. Either you can uninstall the add-on or disable it with a registry fix.

Here are the articles I found on it:

Article 1

Article 2