Click the Startup tab and you'll see a long list of everything that runs when Windows starts. The System Configuration window that opens lets you change which programs run at startup. Press Windows+R to open the Run window, type msconfig and hit Enter. The first two steps are for Windows Vista and 7, while Step 3 talks about Windows 8 only. Removing unwanted startup programs is easy.
When you install software it will often set itself to run when your computer starts, which isn't always necessary.
So-called startup programs can really slow down your computer and you likely don't even realise they exist. Your computer is probably clogged up by a plethora of spamming software that starts every time you turn it on.
