There's no opening up your computer, and no installation woes. The USB port can send and receive data at higher rates than your parallel and serial ports, so USB Netcams tend to show video at a higher quality than cameras that attach via the parallel and serial ports. But, they tend to show video at a lower quality than Netcams that plug into an add-in board on your computer. For most, a USB camera is the way to go. Of course, to use one, you'll need to have a USB port on your computer.
Most computers manufactured in the past few years have USB port.
If you're not familiar with hardware, this can be difficult to do. Generally, the cards can transfer data faster than USB ports, so these types of Netcams display video at a higher quality than USB Netcams. Mostly they've been replaced with USB port Netcams, but they're still around. Although they're easier to set up than Netcams that connect to a card in your PC, they're harder to set up than USB Netcams. Because serial and parallel connections are slower than a USB connection, the quality of their video isn't as good as a USB connection.