depends on what you want to do, if you have a system with always on access you can setup Remote Desktop, it is part of Win XP, Vista and 7, just get your IP before you leave and use it as the connection address, then when you connect it is like you are sitting infront of you screen at home. Just remember to Lock or Log out when you are finished.
If you need more functionality Log Me In is good, you have to install a host on your system and leave it running but it's light weight and easy to use, with some very nice functionality. -
https://secure.logmein.com/US/home.aspx
I have also used VNC, which is very functional and also requires a host application running on the system you wish to access. -
http://www.realvnc.com/
There are free versions of both, so check them out, like I say it depends on your needs, I kind of stick with Remote Desktop, just because it is part of the OS and does not require an additional service to be installed and running, it uses Terminal Services host so unless you disabled that in your system it should be running by default.
I do most of my support work for users remotely so I have quite a bit of experience with it, if you have problems or more questions let me know.