The only problem with the Remote Assistance in XP is (I believe) one of the computers has to be running XP Pro and not XP Home. That is at least true for the "mstsc" application (Start > Run > type in "mstsc" > Enter/Ok).
Another good program is called
RealVNC. I used this for awhile since I used to only have XP Home edition and I wanted access to my computer from anywhere. The free edition has very limited features, but I just noticed someone has made a program called
TightVNC which seems to be an open-source version of RealVNC with all the same features. I haven't used this yet, but it looks pretty good.