Set the List Box's RowSourceType to 'Value List' then in the forms 'OnLoad'
event use:
Set fs = CreateObject("Scripting.filesystemobject")
Set f = fs.getfolder("path to your folder with a trailing \")
Set fc = f.files
For Each fl In fc
strng = strng & fl.name & ";"
Next
Me.[List Box Name].Rowsource = strng