Make sure there is only one AV program running. Two at the same time can make a P4 into a 486sx. This can happen when installing an update. Same is true of AV and spyware protection.
If using an AV with a live update funtion this can cause a slowdown if it trys to update while not connected. Connect and force updates until you are told there are no more. Norton does not always fully update with one connection. Most times it warns you but not always.
Check the running processes and see what the heck is running. Delete from startup those un-needed item. As lot of software wants but doesn't need a position in the sys tray.
Check online for an updated driver for the graphic card because as you said that can be the problem.
There that should keep you entertained for a few hours. 
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