How to access my desktop remotely from my laptop upstairs?

Hello. I have a desktop downstairs connected via patch cable to our wireless access point (desktop doesn't have a wireless NIC). I also have a laptop upstairs that connects to our wireless internet. How can I access my desktop PC from my laptop, or vice versa? I've tried XP's wizard, but I can't get it to work on either PC. Both computers go through the same router/access point, so it's surely possible?