I would be very very surprised if your computer ever booted up "instantly." Even if you have set quick boot it will still take a short while to load everything, especially Windows. As said, over time junk accumulates on your computer. Even deleted files are not fully deleted until the part of the hard drive that contained those files is overwritten. When a file is deleted only the header is deleted, that is the part of the file that tells Windows, or any other operating system, that that file is no longer in use and so should not be booted. Computers though are pretty stupid animals. The 1s and 0s that originally made up that file are still there, until overwritten, and your computer still has to check those redundant 1s and 0s. This stuff builds up and the only way to clean a hard drive completely is to perform a clean installation, as one of the things that kalju has asked you about.