One of my most trusted and relied upon tools is LogMeIn. I use this almost daily to remotely control computers. I install LogMeIn on all of my clients' servers so that I can remotely access them as needed. I also install it on my clients' desktop and laptop computers so that I can help them with problems remotely instead of scheduling a service call. It saves a lot of time and gas.
LogMeIn is very secure and works very well. The free version does everything I need in most cases. There's also a paid version that enables additional functionality such as remote printing and file transfer.
LogMeIn works closely with Intel and they have developed technology that allows you to remotely control a computer through LogMeIn below the operating system level (out-of-band). That means that you can remotely get into BIOS and change settings and even remotely install an operating system. The technology is available on some computers and motherboards now and will be available on more models in the future.
Besides the version that you have to install on the computer that you want to remotely control, they also have a free beta service called LogMeIn Express that lets you hold online meetings and remotely assist people who don't have any other remote access to their computer. It even lets you transfer files!
Check out LogMeIn and LogMeIn Express: