What file is it? What OS are you using?
If you can’t delete it, it’s probably because the file is currently in use (“running”). If you want to delete the avast folder, it would be better to uninstall it (from the Control Panel / Add/Remove Programs) than to delete it directly.
The file is the shell extension of avast (i.e. the “context-menu” handler).
The file is registered multiple times in the system registry (to show up the avast item in the context menu for any file and folder). If you are familiar with registry editor, you can try to delete the avast item in HKCR*\shellex\ContextMenuHandlers and HKCR\Folder\shellex\ContectMenuHandlers and restart your computer.
However, I would really recommend to use the usual uninstallation process instead…
Sorry if you are “going out” of avast…
But, if igor´s answer seems “difficult” to you, try a registry cleaner tool (jv16 PowerTools at www.jv16.org). I do recommend that - at least before running it - you try what igor says: usual unistallation process…
that won’t work:
a registry Cleaner only removes “blind” entries from the registry AFTER you have removed the file in question
and i don’t know if the Startup or installed software approach would be so easy/advisable for Newbs…
if usual uninstall won’t work, try:
a) renaming the .dll-file in a Dos-Box in SafeMode (F8-Boot); then or maybe after a reboot you can delete it. Afterwards, Regcleaning would be a good idea