Don't Forget to Restart your Computer
Restarting your computer a few times each week is an important way to keep your computer healthy and running smoothly and quickly.
One of the major reasons your computer slows down is that its Random Access Memory (RAM) is being used. The operating system, as well as the programs you're running, all use RAM. However, there's only so much of it, and it can only be accessed so fast. If your computer is trying to use a lot of RAM (often more than is available), it slows down. It then creates temp files on the hard drive to act as extra memory, but less efficient RAM. This, in addition to having many applications running, makes the hard drive run slower and you will start experience general poor performance.
Closing some programs should free up RAM space, but memory leaks may have occurred. That means that the program may have accidentally taken up RAM that it didn't/couldn't free up when it closed. If you restart the computer, all the RAM is cleared out. Now you have more available RAM, so your computer can run faster.
There are other problems that could be fixed by a restart, too. For example, if a program somehow begins to use a huge amount of processor cycles (each cycle consists of a calculation, and all of these calculations make your computer "compute", aka work), restarting your computer can give control back to the operating system, instead of the greedy program.
Restarts also complete the installation of software updates and security patches, like Microsoft patches which are installed automatically on PCs each month.