Until recently my favorite tool for remote control of the PCs of Dad, Mom and Father-in-law was LogMeIn. A great service and the free edition is powerful enough for the daily problems. But you need to go there at least once to install the logmein-client so you can later remotely control it.
Now with Crossloop there is another option if you need to get access right now. You and the person in front of the remote PC install and start a piece of software, enter a common security key and you have access to the desktop. CrossLoop is an innovative piece of software, built over TightVNC technology, that allows you control the computer desktop of a remotely located friend using your own keyboard and mouse.
The only condition is that both of you must be connected to the internet at the same time. The CrossLoop option comes in handy when the problem is too complex (like you want to diagnose some Windows Error message) or your friend needs assistance for installing some software on his machine. You can take control of his machine and fix the issues while he has gone to grab a cup of coffee.