I'd rather use Linux as host system, and Windows XP as guest, maybe with host-only network, but I can think a quick and dirty solution for your case anyways.
In short: install VirtualBox, install a Linux guest, get an USB network adapter, and configure VirtualBox to connect that piece of hardware directly to your guest machine, and disable any network card accessible to your Windows host.
In detail:
- Download and install VirtualBox on your Windows XP host system. It's free and open source. Download and install also the proprietary VirtualBox Extension Pack, that, among other things, lets you connect USB devices directly to your virtual machines.
- Create a new virtual machine, and install a GNU/Linux system on it. Try Lubuntu for a very lightweight but still very usable system.
- Install VirtualBox Guest Additions on the VM. Best way to do it may depend on your guest OS. For example, if you are running a recent version of Ubuntu (or any derivative of it, like Lubuntu), you can install them from regular repositories.
- Once you're sure your guest OS is running fine, disable any networking hardware accessible to your host system. For example, disconnect any ethernet cable, or disable wifi from your mainboard BIOS.
- Get an USB network adapter, or an USB Wifi adapter that is known to work with your Linux OS. Nowadays you can buy such a device with less than 20€, and most of them are supported by Linux out-of-the-box. We are going to connect this device directly your virtual machine.
- Shut down your virtual machine, and open its VirtualBox settings window. Be sure your USB network adapter is disconnected. Now we are going to tell VirtualBox to connect that USB network adapter directly to your VM. Here's how:
- Go to
Ports
tab, and look at USB ports settings. There should be an icon with a USB cable with superimposed a green "plus" sign.
- Click on it: this will list the USB devices currently available. It may appear your keyboard, your mouse or your webcam. Take note of the devices you are seeing right now.
- Now dismiss that list, and connect your USB network card. Tell Windows to ignore this new piece of hardware.
- Back to Virtualbox, click again on that USB cable icon with the green plus sign: now you should see all the previous devices AND your new device. Click on it to add it to the list of devices that will be connected directly to your virtual machine.
- Go to
Network
tab, and disable the VirtualBox-provided network card. This way we added an USB network adapter that can be directly connected to the Internet, and disabled the virtual network adapter that use your host OS to connect to the Internet.
- Now boot your guest OS. You should see that its old network card is not available any more, but there's a new network interface. Most of Linux systems are "smart enough" to cope with this change of network, and to connect seamlessly to the network.
Warning: This should work, but I never tried all these steps myself, I just connected once an USB wifi adapter to a Linux guest running on Virtualbox to debug some wifi problems.
You can expect problems with Windows trying to use your USB network device anyways, even if you instruct Windows to ignore it, making it difficult for the virtual machine guest to access that hardware.