a scroll bar with mouse or without mouse?

with mouse it is a little more complicated than without because there is a need of 2 adjustments, one for the "cursor of the scroll bar" to "first line of text to display", and one for "first line to display" to cursor of scroll bar". if you don't do that, your cursor will not be accurate to the real position, like the volume cursor in vlc 0.8.6i for example, where the cursor is one pixel behind mouse cursor above 59%

without is very simple, take the keys from keyboard, and change Y coordinate of the cursor.
this coordinate can be a percentage of the total scroll bar, or just a relative coordinate to the top of scroll bar, with a limit wich is "scroll bar lengh" - "cursor lengh".

did you tryed to draw your scroll bar theory before to ask here?
because just with a single drawing, showing variables for the scroll bar and math relation between all them will give you the solution.

Why not just have a buffer and write the names to buffer from the DTA Buffer.
The names are 13bytes long so copy from the DTA Buffer to your main buffer and add 13 byte to the pointer untill your buff is fill or theres no more files.

That way you just move the start offset in the buff up and down *13.
And display them to screen.

BTW, I think file name are only 11 bytes (8 byte file name, and 3 byte extension)

---all versions, documented fields---
15h BYTE attribute of file found
16h WORD file time (see #01665 at AX=5700h)
18h WORD file date (see #01666 at AX=5700h)
1Ah DWORD file size
1Eh 13 BYTEs ASCIZ filename+extension