How do I retrieve all different items in listview

First of all, to solve such problems efficiently, you can use data structures based on hash code and buckets, used in .NET BCL for implementation of some collections such as associative arrays. These algorithms offer the time complexity of O(1) for search of "already added item". Please see:
http://en.wikipedia.org/wiki/Associative_array[^],
https://en.wikipedia.org/wiki/Big_O_notation[^],
http://en.wikipedia.org/wiki/Time_complexity[^].

If you did not have to find counts for each item, you would use HashSet; collect items in an instance of a set, checking up if the item is already in a set before adding. I think this should be clear. Please see:
https://msdn.microsoft.com/en-us/library/bb359438%28v=vs.110%29.aspx[^],
https://msdn.microsoft.com/en-us/library/bb356440(v=vs.110).aspx[^].

If you also need to calculate counts of occurrences, you need a similar collection based on key-value pair, such as Dictionary. Here is your algorithm: keys should represent your items (not necessarily strings, any objects for which you define equivalence and hash value), values should represent counts. Each time you get new item, you first check if it is already in the collection. But don't use ContainsKey, use TryGetValue, so you can check the present of the item and get the item if it is in the collection. If the item already added, increment the value part corresponding to it in the dictionary, its count. If not, add one with the count of 1. That's all.

Please see:
https://msdn.microsoft.com/en-us/library/xfhwa508%28v=vs.110%29.aspx[^],
https://msdn.microsoft.com/en-us/library/kw5aaea4(v=vs.110).aspx[^] (no need to use it),
https://msdn.microsoft.com/en-us/library/bb347013(v=vs.110).aspx[^] (use this for the check).

—SA

I simply had to enumerate through an array of values which had a bunch of options in them, enumerate the ListView items, create a dictionary and store keys and values in that. Then the key and value is added to the ListView to get the count:

Private XItems As String() = {"Windows 2000", "Windows 2000 (XBOX)", "Windows XP", "Windows Vista", "Windows 7", _
                                 "Windows 8", "Windows 8.1", "Windows 10", "OS X", "Android", "iOS", "Linux", "Ubuntu", "Other..."}
    Private DICT As New Dictionary(Of String, Integer)
    Private Sub LVLoad()
        DICT.Clear()
        ListView2.Items.Clear()
        For Each ITM As String In XItems
            DICT.Add(ITM, 0)
        Next
        For Each ITM As String In XItems
            For Each XITM As ListViewItem In ListView1.Items
                If XITM.Text = ITM Then
                    DICT(ITM) += 1
                End If
            Next
            ListView2.Items.Add(ITM).SubItems.Add(DICT(ITM))
        Next
    End Sub