If you're not going to use the computer for some time or you want to turn it on right before you get home, WakeOnStandBy should be useful for you.
It supports most versions of Windows, but if it can't wake up your computer, it tells you so.
After waking up you can leave it as it is, let it go back into standby mode after a while, or set it to run an application using the available array of parameters.
Once the countdown starts, the program stays on the system tray. If you want to check the activation time and the task to be run you just have to double-click the icon.