Actually it's "join (your) XP Pro PC to a domain". Can you explain our situation or needs a little more clearly?
Basically what's needed for a domain to exist is a server running as a domain controller and your pc configured for domain access via Active Directory authentication.
I have a feeling though that this isn't really what you're wanting. If you're wanting to access your files on your XP Pro PC from another pc over the internet, then you would use Remote Desktop Connection, or a software program like PC Anywhere. There is a program that's free from download.com (can't remember what it's called) but it's like PC Anywhere. You install the program on your home pc, then from another pc you log into their site and provide a username and password and it opens up your home pc from anywhere in the world.
Hope this answers your question ... though it was kind of tough because I'm not sure of what your setup is ... meaning if you actually have a domain controller server. So, if you can clarify a little more of what you're wanting, we'll be glad to help. Good luck!