This is the simplest way to perform backups, but it's suitable for people with just one or two PCs. Plug a high-capacity USB hard drive into your computer, and set up a backup program. Windows 7 has one included--Windows 8 will add File History capabilities to the mix--and copious options exist online. If you arrange for automatic backups, so much the better.
Another strategy is to keep two computers in sync so that if one goes down, the other is available so you can pick up where you left off. Again, this option is effective only for very small businesses or in environments where everyone uses the same machine. One big advantage of a sync strategy is that you can set up computers in different rooms or different parts of the building so that if something happens in one part of the workplace (or if a thief steals equipment from there), the other side of the building may still be safe. Check out GoodSync for a solid sync arrangement.
When multiple computers need backing up, a network-attached storage (NAS) system makes excellent sense. A NAS device attaches to your router. You then use included software or your own backup program to back up to the NAS periodically. One drawback: Often, the backup software included with these drives is limited, and backup traffic can be so heavy that it floods your network. Check out the WD MyBook Live series for a great small-office NAS.