This is a long standing problem with Outlook express and some Anti-Virus programs. Microsoft are aware of it. I spent more than one hour on the phone with them in Feb./March of this year, they did not have the answer then, except to tell me to change anti-virus programs. Don't think they have solved it yet. The hot favourite is Freeserve and AVG mixed together, but does not happen to everyone.
The message just means that the last area of memory that you accessed using Outlook express is refusing to let go of the memory, hence the message. That bit of memory cannot be written. If you use "Outlook" or another email program, you will not see this message. Sorry I don't have the answer, I can only repeat what Microsoft said to me. j.
We had a long running thread on this, think it was under the username of scooby43. It had about 35 or more replies to it, and we still could not solve it. Just done a search but can't find anything. I believe that scooby cured his by changing from AVG to another anti-virus.
I had changed ISP to Freeserve at the time and thought it was their fault so changed back to AOL all this was within a 10 day period. By changing back to AOL and using their email program it cured the problem for me, but many others are still having this problem. j.