In reply to 5tormin':
I don't understand how this can help him.
For Baldwin: How are these devices set up? Internal wireless card on the desktop/laptop or does the desktop use a usb dongle?
TBH it sounds like an issue with the way the network sharing is set up. This goes a long way to explaining why things are screwing up desktop > laptop but not the other way around, winxp doesn't play well with later versions of windows in a lot of networking situations. A couple of things you might check though:
Do they run the same antivi (if any) with the same settings - they both allow all network traffic?
Are both machines in the same workgroup?
Is the windows firewall enabled on both or a mixture?
Can't advise much more than that really. This is literally the worse combo you could run, outdated xp and shitty 8.1 =/