How can I use Remote Access from my home to my office, and from my office to my home?
I am using Windows XP in the office and Windows 7 at home.
Follow the guide on this. Note, that if you do not have Windows XP Professional, then you have to download it at the link provided on the page.
Of course, you can configure a VPN or open a port in your firewall...
But you might want to consider TeamViewer; free for home use, not for commercial.
Or LogMeIn; basic services are free.