Microsoft issues a regularly-scheduled set of security patches On the second Tuesday of every month, and within the next couple of days almost most of the computers reboot automatically because When Windows Update installs certain types of updates, a reboot is required so that certain software components can be updated while they're not running. you can prevent your machine from rebooting using one of this method,

First Method: you can configure Windows Update to download but not install updates automatically. To do so Follow following procedure

Step I) Go to your control panel and than to System and Security.

Step II) Go to Windows Update.

Step III) Click on change settings on left hand pannel.

Step IV) Choose "Download updates but let me choose whether to install them" from the "Important updates" drop-down list.

Second Method: You can tweak your registry not to reboot your PC after update assuming you have followed above procedure,

follow this tweak to change your registry settings