You can find third party software.
e.g. Bells & Whistles -
http://www.emailaddressmanager.com/o.../tracking.html
You could try some VBA. This is the tricky part that generates the unique number.
http://www.vboffice.net/sample.html?...5&cmd=showitem
Code:
Public Function GetNextID() As Long
Dim lCurrID As Long
Dim File As String
File = "C:\Sample.txt"
lCurrID = Val(ReadFile(File))
lCurrID = lCurrID + 1
WriteFile File, CStr(lCurrID), False
GetNextID = lCurrID
End Function