One of the biggest reasons people leave their computers on is because it takes practically forever to start up again! The biggest culprits behind slow startups are autorunning programs.
Autorun programs are set to boot when your operating system boots. Essentially, you end up with dozens of processes and applications all trying to start at the same time, which creates a huge logjam of activity.
But the truth is you don't need most of these programs on all the time - some of them you might not use at all! Tracking down autorun programs and preventing them from turning on automatically isn't the most intuitive thing to do in Windows.
Thankfully, you can download Autoruns. It's a free program that tells you every program turning on upon startup and what order they proceed in.
Just download Autoruns, extract the file to your hard drive and run it - no installation necessary. You can extract all files or you can just extract Autoruns. You won't need the file Autorunsc unless you want to run it using command-lines.