run the avast virus cleaner to kill mydoom. avaliable form the avast home page. avast can be set to update itself automatically others will tell you how to do that
Maybe, if you already has an infected file in your computer with a new virus that was not detected by the ‘old’ virus database, then a full scanning will be very good. But, the majority of the times (almost ever), it won’t be necessary if your on-access settings are well configurated. I suggest a full scanning each week or twice a month. In the Pro version you will be able to schedule scannings when you are not at the computer. The cleaning could be set to automatic there too.
Better is delete the file or send it to Chest. Look the Mrs. Yuna suggests and run the avast! Cleaner.
Everytime that you are not sure, you can send the files to Chest and ask for help .
Wellcome to forums.