You will need to do these things:
1. Set up DDNS on your office computer.
http://www.dyndns.com/services/dns/dyndns/2. Set up port forwarding on your office router to forward port 3389 (RDP port) to your office computer.
3. Setup a secure user name and password on your office computer and enable remote desktop in the operating system, if it is not already enabled.
That should do it.
There are commercial products that also accomplish the same thing, if you are concerned about security, or are uncomfortable setting things like port forwarding on your router up.
Specifically I know of a lot of business use solutions from citrix. See:
www.gotomypc.comAlso, microsoft small business server does some of this out of the box.