the little red disk

iv got an slightly aged computer, which has become riddled with minor viruses, i want to put my pc back to its factory settings and start like the first day i turned it on so this time i can install all the required protection software. my question is that i have done this before with this computer, using a little red recovery floppy disk.
im no wizz with computers and all i know is tht i used to put this floppy in and start it up and a while later the pc would start up again just like id taken it out the box.
but this disk seems to have gone walkabouts. so does anybody know where i might be able to get one of these recovery disks or create one.
i do feel it would be a waste if i had to discard this pc so any reply would be appreciated…