If your Internet Security (Internet Options/Security) is set to High, I know some people haven't been able to use P2P with Vista. Try Medium with cookies enabled.

Be careful with installing the XP software on your laptop. If the software came with your old desktop, it may only work on desktops. Dell is notorious for pulling this.

Also, have you tried setting LW to run in Win2000 mode? Apparently that's helped out some folks.

In Limewire, what Listening Port and Router Config are you using?

Good luck!