If you're using Symantec/Norton AntiVirus, try disabling the
Office Plugin that comes with it. See http://support.microsoft.com/?kbid=329820 "How to use Office
programs with the Norton AntiVirus Office plug-in" for more
information.
For those running Norton 2006, easiest to right click on the Norton Internet
security on the Taskbar, click on the options and then turn off the office
add-ins.