Dim your variables with the correct type.
Set fs = CreateObject("scripting.filesystemobject")
within your loop. That should be created once I would have thought?, and reused within the loop.
Use the Macro recorder to see what code you need for various tasks.
To format the cell as hh:mm:ss you would use
Selection.NumberFormat = "hh:mm:ss"
but you would put the value 0 in the cell.
I would have thought you would be using the same sheet for this, so you could set the formats manually and not repeat them every time the code runs.?
Indent your code so you (and anyone else) can easily see the structure of it.