theres alot of confusion and myths about the laws, here are the actual laws about it broken down in plain english from the antipiracy act:
U.S. law allows you, the owner of a software product, to have three (3) copies of that product at once: the original, an installed copy, and an archive copy. The law does not give a definition to these terms. So, in the sense of a ROM image, you can arguably say that the image on your harddisk is being used to play Zelda 1 is the “installed” copy, the cart that you bough 10 years ago is the “original” copy and a copy of the ROM image on a floppy disk, tape, website or other medium is a “back-up” copy. But, only one of them can be in use at once. So, playing the game on your NES and on the PC at the same time is ILLEGAL.
If you got a copy of a software product as a **gift** (see section below) it is ILLEGAL to duplicate it. Reason: you are not the original copyright holder
AS OWNER you can give 10 people copies (purchase price must not exceed $1000) every 180 days. ROMS sites that don't control how many people are downloading their ROMs can get into some deep trouble if someone can prove that more then 10 people are downloading them everyone 180 days.
unauthorized modification of software (ROM images) that you legally own is illegal.
24 hour law limit was removed during the 1997 anti-piracy act.
The law does not state how you can make an archive copy of your game.