You say you have a numeric field, standard, 2 - I think you should change standard to fixed, forcing Access to always display 2 decimals when you view the data.

In your Form_Load() event, add code to set your textbox to "0.00":

Private Sub Form_Load()
Me.Text1.Text = "0.00"
End Sub

Regarding your insert statement, use:
INSERT INTO <table> VALUES(<num>) for numbers, and
INSERT INTO <table> VALUES('<string>') for strings.

Are you using ADO, and if so, why not share the code with us, maybe I'll be able to spot the error more easily...

Last, in order to let the user replace what was previously typed in your textbox, you should look into writing code in your control's _GotFocus() event:

Private Sub Text1_GotFocus()
Me.Text1.SelStart = 0
Me.Text1.SelLength = Len(Me.Text1.Text)
End Sub

Best regards
Franz Hemmer