I am at client doing an Exchange 2000 to Exchange 2003 migration. When I was doing the in-place upgrade ,I came across the following error message:
"cdoex.dll used by another application"
I couldn't continue the upgrade process before of this message. After doing some troubleshooting it came out to be the MOM agent was causing this issue. I stopped the MOM agent and everything seems ok and completed the upgrade without any issues.
Lessons learned: Stop all monitoring and management services before you do any upgrades. It is a good practice to stop SMS, MOM, Antivirus etc. services before do any upgrades..