In the majority of cases you will not be able to check the file which is currently using another program. This file is locked, and the operating system will not allow you to access it. It applies to both the operating system, that uses the files on behalf of itself, and the working programs. The latter need not be seen on the screen of your computer every time. However, if they are working and using the files, AVAST32 will not be able to check these files.
If you are working with the operating system which supports safety aspects on the level of files, and if you are using the system of files which also supports this characteristic (Windows NT with NTFS), you must have sufficient rights to be able to check individual files. If your rights are not sufficient, the file will remain unchecked.