Set CPU Affinity For 4 Cores Machine?

In my application I has a feature to set CPU application affinity for the application. I have a combo box with the following items:

Item 0: "ALL" -> It's point that if select the application will use all available CPUs
Item 1: "1" -> For set application to use first core
Item 2: "2" -> For set application to use second core
...
Item N....

Where N is the total numbers of CPUs/cores available in the system.

Now when the user click in one of this I execute this

Private Sub comboCPUAffinity_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles comboCPUAffinity.SelectedIndexChanged
Dim num As Integer = comboCPUAffinity.Items.Count - 1

[Code]....

This works well in my vista 2-cpu machine, and also in 1 core machines. The problem is that I have release the application and has a user who has a 64bits vista 4 cores machine, and when execute the above code the application causes a buffer overflow. This is maybe because I have something bad in the code which can not test because I have not the machine to test.