On any Windows OS other than 2000, try Start>Run and type msconfig. Press OK.
If the machine is already in selective startup, remember this point for later.
Go to the Startup tab. Knock out everything and press OK. If the problem goes away, try reenabling half the items. Eventually, you'll narrow down what it is that's doing this.
If that doesn't work, knock out everything under the general tab under selective startup until you narrow down the location of the application. The point is to keep eliminating stuff until the problem goes away. Then you'll be able to narrow the search to a specific item.
To reset things, if the machine was not in selective startup, go back to the general tab and choose normal and press OK. Otherwise, set things back to the way they were. Press OK and the machine will reboot.
If you exhaust this forum for suggestions, and you're still stuck, give us (Microsoft Support) a call at 425-635-7056 and we'll get to the bottom of it for you.
Note: If you haven't used your two free incidents of support that come with Office, it won't cost anything.