I have seen these errors a few times, where the Application Event log shows ID's 7076, 6398 and 6432 with the message "Attempted to read or write to protected memory". Microsoft has located the problem to be related to IIS and have issued a fix at http://support.microsoft.com/?id=946517. It can be installed regardless of MOSS/WSS service pack installed. Also see the following article for further details - http://sharepoint.microsoft.com/blogs/fromthefield/Lists/Posts/Post.aspx?ID=47
Update (courtesy of my colleague Virinder Aulakh): You get an unspecified SMTP error when you try changing SMTP settings in IIS after applying the hotfix (KB946517) to resolve Event ID 6398 errors. The problem is described in detail here http://blogs.msdn.com/vijaysk/archive/2008/04/14/issue-smtp-configuration-unspecified-error.aspx and the subsequent Microsoft Knowledgebase article is here http://support.microsoft.com/default.aspx/KB/950426.