spats,
Welcome to the forum. Don't worry about being a novice. We can teach you what you need to know.
Firstly, You stated that you just installed Avast. What build of Avast are you using? If you are not sure, right click on the A ball in the tray. Go to About. Up on top it will give you the version which will should read "build June 2003, 4.0.235. If you did not update Avast after installation or if it did not prompt you that a program update was available, then you are working with an older build which might contain some "bugs". At this point, I would right click on the A ball in the tray....go to Updating.....and click Program update.
This will update your version to the latest build, and at the same time update the database. (Program updates always will update the virus database whereas Database updates will only do the db)
Rerun your standard scan, setting AVAST to the STANDARD scan... Thorough... and scan Archives.
To do this, go back to the Main interface. Left click on the drive icon next to the Resident Protection. This brings up a new window where you can move the "slider" to change it to Thorough and also check Scan Archive files.
This is a peculair behavior for Avast. So let's get things current and try the scan again. If this does not work, then a corrupted file somewhere within the program could be the culprit.
Do not uninstall Avast at this time!
OK?