What marks xls document as malicious when oppened in Excel 16?

I collect data trough Excel document(xls, Excel 2000) and there are like 300 people across the country who fill these excel files and send them to me.

I have an Access 2000 application that reads those 300 xls documents, but it fails to read some of them.
When I open the faulty excel files in Excel 16, Excel 16 displays a red error message bar which states that there might be malicious scripts inside the xls file so it is oppenned in Protected View.

What could be causing this problem? Why the files are marked as malicious?

Can I somehow improve, and somehow handle this sort of error in Access 2000 when it parses them?

Or just use VBA exception handling and print which files are faulty, and then open them manually in Excel 16 and save them as not malicious?

how are you processing the .xls files?
possibly you could use ADO to process the files, that should not care about any malicious intent

it would seem a little strange that some can be processed and others can't, do the exceptions possibly have some macro code?

It sounds like there's VBA code that's being embedded into those files that's set to auto run or something. There could be something on their system that's running that's causing this issue. Your system is doing the right thing in stopping it from running.

-tg