When you say you, "want to get wireless back on my old desktop with Windows XP", I assume that at some point you did have a wireless device installed. Was it either of the two devices you are currently considering? If so then the driver for that device is probably still present on the XP machine and shouldn't involve anything more than reconnecting that device.
If not then basically you'll want to determine the Device's model name, model number and Rev # if present and then go to the manufacturer's website and obtain the applicable driver for your device and Operating System. The driver that is present on the Windows 7 machine probably wouldn't be of any value to you in so far as the XP machine is concerned (unless of course it's one of those one size fit's all drivers that includes XP or allOS in its' name). As far as one being better that the other, I really couldn't say, although Belkin may be a little more recognized generally it actually depends on the chipset present on both devices.
