We have the same issue with Exchange Server 2003 SP2 running on Windows 2003 SP1.
Store terminates with two events 7011 (Timeout (30000 milliseconds) waiting for a transaction) and 7035 (MSExchangeIS service stopped unexpectedly).
This happens only when some one attaches a document over OWA. We have also noticed that, for every such event a popup of DEP (Data Execution Prevention) also thrown.
Temporarily we have disabled OWA attachments, but need to fix the problem quickly as possible.
Any help will be highly appreciated.