Vista handles networking just as well as Win XP, if not better. I work with both wireless and wired networks in a Vista environment, and I know of no real reason why Vista should be worse than XP in this context. If you want to tell us what the problem is I'm pretty sure we can help you fix it.
If you're really sure that you want Vista off your drive and XP on, it can be done, but you'll need a CD that boots into an MS DOS environment first.
Here's what you do:
1. Start your computer with the Windows XP installation CD in the drive (you may have to change your BIOS settings to make your CDROM drive the first boot device instead of your hard drive).
2. In the setup type R to repair an existing installation
3. Enter "fixboot C:\" (where C:\ is the drive with Vista installed, replace C with any other letter if applicable)
4. Enter "fixmbr C:\"
5. Enter "EXIT"
6. Then close the Windows XP setup and restart your computer with the bootable DOS CD
7. At the command prompt type format C:\ (Be warned - all your data files on C:\ will be removed, so make sure you've made a backup before doing this)
8. Now restart your computer with the Windows XP installation CD in the drive, and the installation will start normally.
You can download an MS DOS boot disk if you click here
You'll need to put the files onto a CD, and if you don't know how to go about all this make sure you read the 'How to' section on the link I've given you.