Loop To Make Default Name For File Before Saving?

I am creating a button to save text files (Visual Basic 2005 and Windows XP). I made the following loop to give a default name for the file before saving:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

'This loop is supposed to name the file "file10.txt", and if such a file exists it is
supposed to name it "file20.txt" then "file30.txt" etc.

Dim myFileName As String
Dim i As Integer = 10
myFileName = "D:filesfile" & i & ".txt"[code].....

However, the app gets unresponsive only after one round in the loop (when the file name is "file20"). The loop gets stuck at the following line:

If IO.File.Exists(myFileName) = True Then

This line works only once and after that it becomes unresponsive.