I've had consistently good results with the free version of Logmein. I use Haruka (same company) for moving files around. Logmein doesn't care whether you have fixed or dynamic IP.
I've also used GoToMyPC, VNC, VNC over SSH and XP's own Remote Access service. They work, too, with varying levels of farting around with the router (except GoToMyPC, which uses an intermediary to set up the connection, like Logmein). I found a couple of applications where VNC can't click on the buttons for some mysterious reason, and Logmein doesn't like one of my video window applications.