Excel 2007 & 2010 Static Random Unique Alphanumeric string across multiple sheets

Hi,

I am new to VBA and wanted to make an excel file. I need to create an excel file with 15 sheets and each sheet will have 6 columns;

1. Member ID (Mandatory)
2. First Name (Mandatory)
3. Last Name (Mandatory)
4. Gender (Mandatory)
5. Date of Birth (MM/DD/YY) format (Mandatory)
6. Member Code

All fields will be blank initially. Once a user enters/selects a value in the Member ID, First Name, Last Name, Gender & Date of Birth column, a Unique, Random, 15 character alphanumeric string (only upper case alphabets, no lower case alphabets) should be generated in the Member Code column.

The value should not change even when data for next row is entered. After saving the excel workbook and re-opening it, it should retain all the unique random 15 character strings generated.

I found a good function that can create a 15 character string as required, but I am unable to make it unique within the sheet & across the sheets in a workbook.

The function is taken from this site
http://www.ozgrid.com/forum/showthread.php?t=19314

I modified the function slightly to suit my needs and changed it to this

Function Pwd(iLength As Integer) As String
    Dim i As Integer, iTemp As Integer, bOK As Boolean, strTemp As String
    '48-57 = 0 To 9, 65-90 = A To Z, 97-122 = a To z
    'amend For other characters If required
    For i = 1 To iLength
        Do
            iTemp = Int((90 - 48 + 1) * Rnd + 48)
            Select Case iTemp
            Case 48 To 57, 65 To 90: bOK = True
            Case Else: bOK = False
            End Select
        Loop Until bOK = True
        bOK = False
        strTemp = strTemp & Chr(iTemp)
    Next i
    Pwd = strTemp
End Function

I can throw in some conditional statements to not run this script if the previous columns are empty, but I am unable to make it unique.

Kindly help me out here.

Thanks in anticipation !!!
Vignan