Older computers can gain Wi-Fi functionality through the addition of a peripheral wireless adapter. Wireless adapters come in two main flavors: internally installed peripheral cards for desktop computers and external USB adapters for all computers. Computers without expansion slots like laptops and all-in-one systems are limited to external adapters. External adapters do not require taking the computer's case off to install, making the devices a popular choice even for tower computers with free expansion ports.

Wi-Fi Adapter System Requirements

To successfully achieve wireless connectivity, the computer needs to meet the wireless adapter's system requirements. Fortunately for older computers, Wi-Fi adapter system requirements are on the low-power end and can work with computers in the decade-old-plus age range. A 400MHz Pentium II computer with 64MB of RAM will not be able to run a Wi-Fi adapter that requires an 800MHz Pentium III and 256MB of RAM -- computers over ten years old may require using Wi-Fi adapters manufactured closer in time to the computer. Computers that don't feature USB ports, PC Cards or PCI slots may be too old to add Wi-Fi.

Operating System Requirements

The computer's operating system is a major deciding factor in whether a Wi-Fi adapter will work. Wi-Fi adapters use custom drivers to interface with a computer: if the manufacturer hasn't provided drivers for the operating system, the device will not work on computers running the operating system. Computers made before the late 1990s may not have adequate hardware to run operating systems that support Wi-Fi adapters. For example, if an adapter requires Windows XP or later, a computer running Windows 2000 will need to be updated to work with the adapter.

External USB Wi-Fi Adapters

Almost every computer made after 1997 uses the USB peripheral device connection standard which works with external wireless adapters. Manufacturers make a wide-range of adapters that vary in supported wireless standards and device profile size. USB adapters work by connecting to the USB port and installing software on the computer to use the adapter. USB is backwards and forwards compatible, so any USB adapter can work with any USB-equipped computer regardless of hardware revision. However, a computer running an older USB standard will encounter bottleneck issues with newer Wi-Fi standards. For example, USB 1.0 and USB 2.0 ports can transfer only 12Mbps and 48Mbps, which is insufficient to take advantage of a wireless-N card that runs at 300Mbps. The device will still work, it will just work slower. Older laptops can use PC Cards to add Wi-Fi functionality as well.

Internal PCI Wi-Fi Adapters

Desktop computers can use internal Wi-Fi adapters that connect to either a PCI or PCI Express slot inside the system. PCI and PCIe are not cross-compatible: the wireless adapter needs to match the slot type to work. Internal cards may be a more viable solution for desktop computers made between 1997 and 2001 because the PCI standard has more bandwidth than USB 1.0. Desktop computers made from 1993 on usually include PCI slots. PCI and PCIe cards may require driver installation to work; however, Windows may be able to run the hardware with a default driver.