Notepad with line numbers

Notepad++ is a good replacement and includes line numbers.

Edit: Following your edit saying that you'd tried Notepad++ - you could try Metapad. It's a quick standalone Notepad replacement - it doesn't display the line numbers down the side (like Notepad++ does), but it does display the cursor position (row/column) in the status at the bottom of the window which would effectively tell you what line number you're on when you're editing/viewing.

You can use notepad with "Show statusbar" option checked (you can find it in View menu). And you will have actual line number displayed on that statusbar.

If you have this option "grayed" then you have to uncheck "Word wrap" option in Format menu.

Notepad2 has line numbers. It's not as lean as Notepad but it is very light weight if that's what you're after.