Generally they are the same thread as either the hard drive/case, or floppy/optical drives, usually with just a change in the head if that (there really are only two basic screw threads used in most computer parts).
Try them in the motherboard mount and find which fits, don't force them as one or other should fit fine.
So depending on your "spare screw" collection the chances are you have a bunch of them sat to one side from previous upgrades.
I've got hundreds of the things, as every case I've bought has had them, as have many optical drives and hard drives even OEM's (and going back 15 years or so I bought a couple of bags of assorted screws from a computer fair dealer, back when I was first starting to built/rebuild machines).