Not too much of a surprise.
1. Given that you have had an email alert the details of that alert are also recorded in the emailshield.txt (log) file, why the detection information is detected is what is strange. So that is two down, see #### below on how to resolve.
2. the other entry, not a detection but a missing file is as a result of avast housekeeping.
- Nothing to worry about, those are are files in old virus definitions folders and avast is doing some housecleaning to keep the size used on the hard disk to a minimum. This just happens to have occurred between the time you started the scan and it reaching that old defs folder.
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Effectively you have an email in the Inbox that avast considers infected, so you need to find that email and manually delete it, empty deleted items folder and compact your folders. I can't help with how to find it as you have obscured some of the information. Don't use the Inbox for general storage, treat it as an office in tray read the information and respond to it (if required) and move it to a folder suited to its content. This should make the inbox smaller and easier to find suspect email.
Find (see ~~~~ below) and rename the emailshield.txt file to emailshieldOLD.txt and reboot that should recreate the emailshield.txt. To be able to do that you need to first disable the avast! Self-Defence module; avastUI > Settings > Troubleshooting, uncheck the 'Enable avast! self-defence module' option and stop the Mail Shield, so the file isn't in use. Don't forget to enable the avast! self-defence again when you are done.
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EmailShield.txt location: C:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\report\EmailShield.txt (XP) C:\ProgramData\AVAST Software\Avast\report\EmailShield.txt (Vista, Win7). This may be a hidden location so you may have to change the Windows Explorer, Tools, Folder Options, View, hidden files and folders.