Activating Windows Defender

I have a new Sony Vaio that came with Kaspary Security system - which has 13 days left. It is a Windows 8 Operating system. I know that Windows Defender is disabled - and want to enable it. I have done Windows start menu; Search for Windows Defender. Window appears on the Desktop indicating that it is not active - with no options. I have also done the Administrative Tools in Control Panel - to "activate" - and completed a restart. I still cannot activate Windows Defender. Do I have to Uninstall the Kaspary antivirus before I can activate Windows Defender? If yes, I assume I would do that through the control Panel - uninstall programs?
I am not an advanced computer user, however any help will be appreciated.

You are correct,...you must remove any 3rd party anti virus software from Programs and Features through the control panel before Windows Defender can be activated.

thanks so much - successfully loaded Windows Defender and have run full scan. While scan was running I read (on my tablet) about other security options - Seems strongly recommended that another antivirus software work along with Windows Defender. AVG seems the recommendation. I do not consider myself a Web browser or internet geek but want to make sure that I have all antivirus protection needed. Thoughts on needing an additional antivirus and if yes - recommendation on a free one?
Appreciate the help!

Besides being potential problem, there's no need to have 2 full size AVs, you do not get twice the protection, not even 1% more. What you might do is ad a specialized program against some other things if you feel the need for it, something like Adaware, Spybot S&D,McShield for USB memories and such. You may use third party Firewall if you need more controll and various mallware finding programs but 2 full size AVs are not recommended.