I use PCAnywhere often and it works well. You will need to install one version on the host and one on the remote machine and the machine in your office will need to be on. The good thing is that if your using NT or Windows 2000 you can leave the PC locked so that nobody can mess with it while the remote is controlling it. If the PC is not locked then someone at the PC in your office will be able to override what the remote is doing. In fact you will be able to watch every keystroke and mouse movement the remote is doing (which might be interesting).
I know of another program called Carbon Copy, I know a few people that use this and like it; however, PCAnywhere seems to be the computer industry standard for remote to host control. If your using it over an internet connection and not a dial-up, then you can even set it up so that you can take over from another remote user while they are on. This is great if the other remote user forgets to logoff.