Your computer is likely installing Windows Updates. These are updates, typically released on the second Tuesday of every month, or "Patch Tuesday". To only download and install these updates when you want, go to Start -> Search -> Windows Update -> Change Settings.
From there, you can determine how you want updates installed on your computer (whether automatically or manually). It is important that you do keep the operating system up-to-date, and you instead may want to simply download them all at once by checking for them and installing them yourself. Most of the updates resolve security vulnerabilities.
In the Windows Update area, you should check to make sure that updates are not failing. Use "View Update History" to see what is...
