Even if files are locked against reading you should be able to access them if you are on a recent version of Windows that supports the volume shadow copy service. You will have to create a volume shadow copy and read the file from there.

In any case, if the application has the file open at the time there is no guarantee that the data on disk will be in a consistent state. If you know the application that has the file open (e.g. Word, Excel, etc.) it may be better to take control of the application to save a copy. For this you would need to use the COM interface of the application. There are modules to support some of the more common applications or you can use COM directly, if necessary.