I have seen this error in outlook 2000 but not sure if it would fix 2003. Find the temp directory that outlook is using to store temp files and check the security permissions on that directory. the user would need to have full access to this directory. what happens when you as an admin login to the same pc and do the same thing? if it works then that might be the problem.
Another possibility is that there may be minor corruption in the exchange database and some attachments were lost. Check the exchange server around these times to see if a eventlog entry talking about a corrupt attachment is happening. You may need to defrag and/or repair the database to get this cleared up.