Excel Running Slowly after Windows Update (MS11-021 Office File Validation Add-in)
If you are running Excel 2003 on Win XP/7 and attempt to open workbooks across a network, you may be experiencing a pretty dramatic lag in file openning.
If so, the bug is outlined on the Microsoft website here: See: http://support.microsoft.com/kb/2570623
Fixes can be found here (h/t to steveit.ca for finding these)
http://community.spiceworks.com/topic/146154-ms-excel-hanging-when-opening-a-file-after-recent-ms-security-updates (discussion thread)