It is certainly possible
How to do it is the choice you have to make.
You could use a VPN
You could use terminal services like Remote Desktop or VNC
You could use file and printer sharing ( not secure at all, do not do this)
I acces mine like this using terminal services(Remote Desktop)
It is built in to XP so is an easy choice
From Control Panel - System look on the Remote tab
Tick the box to use Remote Desktop
Setup your router to allow access to the RDP port which is 3389
Now you can connect
Make sure you use strong passwords and renaming the administrator account is
a good idea also
http://www.microsoft.com/windowsxp/using/mobility/getstarted/remoteintro.mspx
http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/583.mspx
Regards
Mark Dormer