Closing windows and tabs freeze my comp

We had a new comp about two or three months ago. At first it was going well, but one day Norton randomly decided to turn off and we didn't notice. The indicatior was usualy red because we didn't have the backups done, so we didn't see it when it randomly decided to turn off all the protections. I noticed after a bunch of downloads and lots of net browsing, and the net computer has been very laggy every since.

Everything goes well as long as I don't close a tab or window. The moment I close one or the other, my whole net lags and crashes. Even shuting down the computer makes it crash. I abolutely need to do "End Program" to be able to turn it off if a window is up when I decide to turn it off.
I'm thinking Norton itself might be causing the lag after the fruck up it did or some sort of virus found it's way in. Whatever it is, is there a way for me to get rid of the problem without having to take my computer to the shop? If it's Norton that's the cause, is there an antivirus I could use that won't cost a fortune and still decently protect my system?