To use a PC card in a desktop system, you must install a PC Card reader in your desktop machine.
This PC Card reader (also called a socket) can take one of three forms:
An external USB device
An internal PC Card socket that mounts in one of drive bays on the computer's front panel
An internal PC Card socket on an expansion card that mounts in a PCI connector, with the slot on the computer's rear panel
Adapters for all three of these types are available with slots for either one or two cards. If you expect to use a PC Card as a more-or-less permanent part of your computer (such as a wireless network interface card), a rear panel slot is probably the best choice because the PC Card in the slot is out of sight and out of your way when you're trying to do something else. But if you plan to add or remove the card more than once every few months, a PC Card slot adapter in a front-panel drive bay or on a table next to the main computer case will be much easier to reach.
Some PC Card readers for desktop computers are still limited to only accepting 16-bit cards. Even if you don't currently use any 32-bit CardBus PC Cards, don't even consider a reader that doesn't accept both 16-bit and 32-bit cards. The difference in cost is insignificant, and there's always a chance that you may want to use a CardBus device sometime in the future.
Any PC Card reader should come with device driver software for Windows XP. If and when you upgrade your computer to Windows Vista, either Microsoft or the maker of the card reader should offer new software in the Vista package or through their respective Web sites.
The only reason to use a PC Card in a desktop system is to use the same device that you're already using with your laptop. If you're buying a new accessory exclusively for your desktop computer, you'll get better performance from a USB device.