It could be so many things that cause the same effect...
It could be that you don't have enough RAM to support all the programs you have running
Or Your Graphic card is taking a dumb
or you have a Virus
Like markoj said, see what is your CPU%, if it is a High number see what process are causing this high CPU%, If they are all process needed to run your computer and the internet, you will most likely need more RAM
If its your Graphic card taking a dump I can't help you there.....
I would Recommend a different Anti Virus Program like goggleeyes30 Said
I recommend
AVG or Avast