Public Function GenerateRandomString() As String
   GenerateRandomString= System.Guid.NewGuid.ToString
End Function

http://codefinds.blogspot.com/2008/10/generate-random-string.html

If you need to generate a random string of mixed characters, this simple function will do it for you. This will return something like this: 80a7a705-9c23-47bc-92b6-2b5c4f9d728d. Which you then can do various string functions to make it the way you want.

Generate a Random String

VB.NET