Why rebooting helps to Speed up computer ?

Rebooting will sometimes clean out temporary files that might have stayed in cyberspace if your computer was not shut down properly. If your system is running slow, and you have not run the Disc Cleanup, and then the Disc Defragment for a while, do that.  It should help. I don't know which Windows you are running, but mine is in the Start Menu, All Programs,  Accessories, System Tools.  If you don't have a lot of memory available in your computer, adding more will help also.  Your instruction manual should show you how to do this.  Memories are different types and sizes.

The following are all the reasons why it is good to reboot your computer every once in awhile.

·    If your computer is frozen, rebooting will "unfreeze it." Sounds simple enough, though the main argument against this philosophy is the belief that rebooting will cause a user to lose work. The reality is, if the computer is frozen, it's already decided if the work is saved, lost, or recoverable. Rebooting is not going to change that, one way or the other. You won't lose work by rebooting that you haven't lost already.
·    If your programs are freezing, but your computer hasn't frozen yet, rebooting will make them run properly again. Often times programs generate fatal errors, for a variety of reasons. When this happens, the programs may close out on their own, or "hang" and have to be closed manually, and frequently will not restart again. Rebooting frees up resources that were previously lost, allowing programs to function normally.
·    Rebooting frees up resources. Your programs are eating up your memory, temporary hard drive space, and other valuable resources. Nothing to panic about, that's their job. But, there is only a finite amount of resources your computer has. When they run out, your computer slows down until it finally halts altogether. Rebooting "refreshes" those resources, and allows you to continue working at an optimal speed.
·    Rebooting fixes problems. When you reboot, your computer performs a series of diagnostic checks and often fixes problems without you knowing. So if your computer is acting strange, often rebooting fixes the error.
·    Sometimes, it just does. We don't know why, maybe it's some mystical mojo, but sometimes rebooting fixes things for reasons we don't know, or just haven't pinpointed.
·    It helps in troubleshooting. Finally, rebooting helps us troubleshoot the problem, and answers one of our first questions: is the problem something that's repeatable. A one time problem, like a program freezing, would not refreeze after rebooting. It may even go away forever. But we don't know the answer until we attempt to recreate the problem, which means starting from the beginning: when you turn on your computer.

One reason I know of is memory. I'm a graphic artist. Many of the programs I use like 3D Studio Max and Photoshop are not only memory intensive, they also tend to hang on to that memory even after they're shut off, due to masses of data being held in virtual memory as well as tying up chip memory. Rebooting clears that out.