ASP.NET Membership provides for the Membership.GeneratePassword method for randomly generating user passwords. This can be customized using the length and number of non alphanumeric characters parameters:

Dim length As Integer = 8
Dim numberOfNonAlphanumericCharacters As Integer = 1
Dim passStr As String

passStr = Membership.GeneratePassword(length, numberOfNonAlphanumericCharacters)