EXEC after FILE - file not exists

I try to copy file to $INSTDIR and then perform operation on it. Unfortuately execution seems to be started before the file is copied. Tho code:

File "proxytrans.ax"

Exec '"$WINDIR\System32\regsvr32.exe" "$INSTDIR\proxytrans.ax"'

The output message is either "file not exists" or "file is used by another process". How to make it correct?

I really don't think this is a problem of line 2 starting before line 1 is finished. Are you sure you're extracting the file to $INSTDIR?

(Might also be related to a very badly sucking virusscanner hogging the file...)