Transferring Values From Form To Form?

Im having a problem when transfering values from form to form.Firstly I have a main menu form,click on a button to go to the dell components form.I choose the components for the computer and when i hit calculate the total is displayed in a textbox for my subtotal. When i press send it will send the sub total from the dell components form to the grand total and display it in the dell sub total in the grand total form.This part works ok, however, i go to the main menu and click on a button to go to the packardbell components form. Do the same again but when i press send, it will send the value to the grand total form in the packardbell sub total textbox but the value in the dell sub total textbox has vanished.Here is the code in the dell components form

Code:
Private Sub btnSend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSend.Click
Dim frm As frmGrandTotal = New frmGrandTotal(txtDellSubTotal.Text)
Me.Hide()[code].....

Finally, the other problem im having is on the main menu form when I click on the view total button I get this error: Here is the code for this bit

Code:
Private Sub btnViewTotal_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnViewTotal.Click
'frmGrandTotal.Show()
Me.Hide()
End Sub