Problem with textbox on xp (but not vista)

I have some textboxes that I do simple math in. The problem is that it works in vista-windows 7, but on xp the textboxes are emty.

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Me.KostOP_TextBox.Text = (Me.SUM_COSTTextBox.Text - Me.Kost_TextBox.Text)

If Me.KostOP_TextBox.Text < 0 Then

Me.KostOP_TextBox.ForeColor = Color.Red

Else

Me.KostOP_TextBox.ForeColor = Color.Black

End If

Me.KostOP_TextBox.Text = String.Format("kr. {0:n2}", (Me.SUM_COSTTextBox.Text - Me.Kost_TextBox.Text))

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It is a field from a dataset (Me.SUM_COSTTextBox.Text), and a tekstbox with a result from a anoter equation (Me.Kost_TextBox.Text). The result is the put into a anoter teksbox (Me.KostOP_TextBox.Text). The result is shown on vista-windows7 but not xp...

Using Visual studio 2008

someone with a good idea....