Create a unique value using Seconds, Milisecons or Ticks [on hold]

I'm developing a C# library with .NET Framework 4.0.

I have a process that generates numbers that must be unique (I can't show you any code because I have signed a NDA).

This algorithm has use DateTime.Now as part of the number, and it also needs a 7 digit random value.

Now I'm using Random and I get repeated values (I have to generate N values). So, I thought that I can use DateTime.Seconds and DateTime.Milisecond, but with both I only have five digits.

How can I make this random value unique?

Maybe I can use DateTime.Ticks instead.