I am not sure I am understanding correctly: "I have a problem with my W7 machine "
Is it on the Macbook Air? Macbooks all have a backspace key in the same place as traditional qwerty keyboards. Look next to the += key, you will have a key labelled with a backwards arrow or "delete" depending on your model. That is the backspace key.
You can also hold fn+backspace to delete to the right hand side.
As noted, the "delete" key that is located where the backspace key is typically located will function as a backspace key even though it is labeled as "delete". In other words, it will delete text located to the LEFT of the cursor (unless something else modifies it such as a key modifier as noted or something in a program much like much older versions of Microsoft Word had a toggle on the bottom of the window to switch between backspace and delete when in Word).
If you have an EXTENDED external keyboard (i.e. with numeric key pad, etc), it will have a second "delete" that will act as what is typically considered a "true" delete key...i.e. it will delete text to the RIGHT of the cursor. This is definitely true of an Apple Extended USB keyboard, but should also be true of any USB extended keyboard hooked up to a Mac that has a second delete between the main keyboard and the numeric keypad as most (if not all) do.