I found it:
Took out the for loop and just added type int indexx and incremented for each file added. Works perfect.
try
{
string[] fileEntries = Directory.GetFiles(tbDirectory.Text);
foreach (string filename in fileEntries)
{
Icon ico = Icon.ExtractAssociatedIcon(filename.ToString());
smallImageList.Images.Add(ico);
lvSums.Items.Add(filename.ToString().ToLower(), index);
index++;
}
foreach (ColumnHeader columns in lvSums.Columns)
columns.Width = -2;
}
catch { Exception ex;}