Unresponsive OS

Hi, I wonder if someone could help me with this problem...

It all started when I accidentally installed AVG Secure Search toolbar which hijacked all my browsers and made Firefox act weird.
I used adwcleaner, malwarebytes antimalware and HitmanPRO to remove it completely. And afaik there is no trace of it right now.

The initial problems ceased but then things got worse and I think it all occurs only when FF is running, but I'm not entirely sure.

a) If I leave my pc alone for a couple of minutes I have set the monitor to switch to stand-by mode. Now most of the times, it wont switch back on even after I move the mouse, press keys on the keyboard etc. Also even if I had music playing, it stops after a while too. So its probably not a display/graphics driver issue.
b) When I'm surfing the web, occassionally everything will freeze and I wont be able to do anything for a short while. Sometimes the OS will unfreeze, sometimes it wont at all and I have to restart the pc.
c) Again while surfing the web, the cursor will start acting funny... it will turn to the icon that appears when typing even if its not on a message box like the one I'm typing now. Or it will switch from the default cursor symbol to the hand symbol and vice versa for no apparent reason.

When c happens, I close all browser windows and open windows task manager which does take a while to load but it seems to stabilize the situation. Or maybe its just a coincidence which means closing down all browser windows fixes the issue.