Right click the avast 'a' icon, select Start avast! Antivirus, that opens up a memory check and once complete it should present the Simple User Interface a little like a media player (you can change skins).
From here you can scan all Local Disks, Folder selection (for a scan of a specific folder, etc.) or Removable Media. You can set the sensitivity of a scan and if archives are scanned or not.
VRDB stands for "Virus Recovery Database"; it was known as "Integrity Database" in previous avast! versions. The aim of VRDB is to help when, despite all the security measures, a virus gets inside the computer and the files are infected. With the help of VRDB, it is possible to repair many infected files (return them exactly to their original state). VRDB is announced by an icon with the letter "i" in the system tray (next to the clock). If the icon is animated, the database is being created right now.
For more information check the avast help file, Healing, VRDB. Windows Start, All Programs, avast! Antivirus, Help. Or if you are in the Simple User Interface, either press F1 or right click on the skin and select Help.