You Can Get There From Here
There are many ways to do this and it will depend on your systems at the office and your willingness to do some basic digging around your router. In no particular order, here are some systems we recommend:
1. Terminal Services. If you have Windows 2000 or 2003 Server, then you already have a way to access the server remotely called Terminal Services. You'll need the client software to access the server. If you have any version of Windows XP, you already have the client. If you're running Windows 98 or 2000 (or, god forbid, 95) you can access the client here. Assuming you have a router to share your Internet connection at the office, you'll need to forward port 3389 to the server to do this.
2. If you'd rather just access a PC than the server, as long as that computer is running Windows XP Professional, you can use a similar method as Terminal Server. You'll just need to be sure to have at least one user on the PC with a password, and select the Remote options under My Computer to set this up.
3. Another good system we often use is VNC. There are many different versions, but we like RealVNC. You can use the free version but we normally use the Personal version or Enterprise as they offer higher levels of security and encryption. Unlike the above two examples, you'll need to forward port 5900 instead of 3389.
4. If all the talk of port forwarding sounds like Greek to you, then consider a service that requires no router setup. My favorite is LogMeIn, although GoToMyPC.com is also very popular. GoToMyPC costs $20/month, while LogMeIn has a free version.