You are assigning the list of instance names to the DataSource - but you have not combined it with the name of the computer.
You need to iterate the set of names in instances() and, for each one, add (prefix) the computer name and the '\' character.
I'm sorry I don't use VB.Net,so can't give you VB code; in C#, using Linq, it would look something like this:
ComboServers.DataSource = instances.Select(n => compname + "\\" + n).ToList()
Hope that helps,
Chris