Boot-up time or start-up time is the how long your computer takes to start up after you turned it on. As you use your computers, numerous files will be stored on your computer along with the ones you manually do, things like your word documents, or videos you downloaded. Files will also be automatically stored as you use it, like cookies and web history being automatically stored on your computer.
Why does it slow your boot-time? It's because the more you have files in your system the more your computer reads files at start up. Especially if you have programs that automatically starts up as you turn on your computer. The more you have start-up programs like that, the slower the boot-time.
There are numerous ways that can help users enhance their boot-time. For instance: Removing all the junk files, as disk space is a key factor in speeding up your computer. You should also remove temporary internet files and empty your Recycle Bin. If you have programs or applications that you rarely or never use, then you should uninstall it as it uses up disk space.
Defragmenting your computer also helps as it cleans up fragmented file that are stored together which makes your files load slower. There are also programs that run in the background that you don't know, these can be found in your task manager, under processes. The higher it is the slower your computer loads as it reads and runs all those at start up. So if you know how to handle these things you should end whatever processes that are rarely or never used at all as it enhances your boot-up time.
You could also configure your start-up which needs at least a vague idea on how computers work as it might cause problems if not done properly. You could go to Start->and type "msconfig" in the search bar without the quotation marks. In the "boot" tab there is a un-ticked box named "No GUI boot", check it and click apply. What happens is that the windows logo will not display when you start up your computer. This saves a lot of resources and time to start up windows.
If you want, and I think you will, you should go to the same "msconfig" panel and go instead to the "Startup" tab. Deselect all of the programs you don't want start automatically when you start up your computer. This is one of the main reasons why your computer takes a lot of time to boot up. If you are contented, then hit apply. This will drastically increase your boot-up time and saves you a lot of trouble waiting.