May be the avast you installed recently may be not compatible or crashed one. I suggest you to switch off the pc, now switch it on and try to press F8 until you got safe mode screen.
After that try to select safe mode and then go to start-->in search box type system restore--> in the list appear select the restore points that the computer works good before installing AVAST. and press ok. Now restart your pc.
check out the results and post back.