Almost forgot, put removable media scan (not just context menu scan) on right-click of a removable media. It's quite a pain when I have to do a 15min+ context menu scan of a 16 GB external harddisk when I can actually invoke a minute of removable media scan--it's just that it's buried in the UI that makes me not to. Yes, laziness, I know.
Or better, rename it to "Flash Scan", "UltraQuick Scan" or something similar to clear the confusion that it's not something that's entirely associated with removable media that you should only scan your FlashDisks using it.
MINOR: Change the tray icon. The orange ball is outdated. avast! has a new logo for almost a year, I believe it's better to use the new lymphocyte thing--to establish a trademark.