|As listed in answer #4:|
Run Malwarebytes to check for an infection your antivirus program may have missed.
Run task manager - processes tab and see what is running. Anything you clearly know does not need to be running shut down, anything you are not sure of, Google or ask us. You can use Microsoft Process Explorer (download from MS) to see more about the processes that are running instead of task manager.
Run msconfig (type in run box or type in search box and it will be at the top of the list) and go to the Start tab. Uncheck everything except Microsoft and your antivirus program from the list (you can add back some later if needed), OK and restart the machine.
Additionally, if you have not been doing this periodically, do a disk clean up and defrag (will not help this, but will help speed up your system, your start up, your program starting, and will reduce errors and improve stability)
These should help you quite a bit. Also if you happen to be running Norton or McAfee antivirus, consider uninstalling and switching to Avast or MS Security Essentials (both free), they use less system resources so your machine will run better.
You have to be a little bit crazy to keep you from going insane.