I think that you're off topic for this forum, which is dedicated to Windows Live OneCare since you are mentioning 40 computers and a server.
However, you should be able to identify the source by locating the backup file on the server and viewing the file properties - specifically the owner of the file.
If you *are* using OneCare, please respond and I'll move the post back to the backup topic folder.
You may want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator