If you have upgraded Windows or, sometimes, when you have reinstalled it, you will be asked to select which version of Windows you want to run when it boots up on a screen like this:
If you only ever use one version of Windows you can switch this off, or you can reduce the countdown time before it continues automatically.
To do this right-click on your 'My Computer' icon and select 'Properties' (alternatively, hold down the Windows key and press the Pause/Break key), select the 'Advanced' page, and press the 'Settings' button in the 'Startup and Recovery' section.
From here you can switch off that selection screen by unticking 'Time to display the list of operating systems'. You can also adjust the time that the screen will be shown before it automatically continues, and you can change the default version of Windows (or indeed any other operating system you have set up on your PC).