If your Excel 2003 files are opening very slow, some up to 4 minutes over the network check to see if your PC has the following update;
“Office File Verification Add-in”
If you have this listed in Add/Remove programs remove it. Be sure to check windows update and hide this update or you will just get it again next time your PC runs windows update.
You can disable it via the registry also
navigate to [HKEY_CURRENT_USERSoftwarePoliciesMicrosoftOffice11.0ExcelSecurityFileValidation] create a new DWORD value named EnableOnLoad and set it to 0