That is the default action if you start the simple user interface (S.U.I.)it does a memory scan unless you untick it.
However if you are using the S.U.I. to scan an IM file you have downloaded this is a very long winded way to do it. You can have your IM program scan the file automatically by giving the path to your AV (in Tech’s post).
If you still want to do it this way you could stop the memory test, see image, but this is a valuable check prior to a regular local disk scan, so I wouldn’t recommend disabling it for everything. You can create a desktop shortcut for “C:\Program Files\Alwil Software\Avast4\ashSimpl.exe” this will launch it without the memory test, but using the regular method of starting avast scan it will scan the memory.
Did you set C:\Program Files\Alwil Software\Avast4\ashQuick.exe to scan your IM files?
Or you’ve done nothing, just set avast! IM Provider to scan your IMs?