This was my 1st build and i forgot to install the shield. I'm worried because something might go into my computer. Is there anyway of putting it in or is there a substitute. The mobo is an asus m4a89gtd
It's a non-issue really. But if you're really worried, follow moto's suggestion, assuming you do have the intended I/O shield.
I suggest not to worry about it too much though, unless you live in a house where cockroaches and mice are large in numbers (to take over the world! ). Especially when you don't have the required I/O shield.
Dust might be the only problem, as long as your gentle when adding usb devices or what ever. The shield does add some protection in the way of rigidity for the ports in the back. As said above, nothing to worry about, just keep the computer clean of dust, and be gentle when plugging things in.