Yeah, it depends on how your XP box is dialing to your ISP.
If it uses a proprietary or specialised dial-in/login procedure, you might be stuck.
But! -- if you can use the native windows dialer to do it, you can use wingate, like ede says.
As for the specifics, I'm really not familiar with realvnc or XP for that matter. I dumped windows when 98 Second Edition was current