A few suggestions:
Like AmberStarfire suggested, see how much free hard disk space you have left. You should be ok with at least 20-50 GB, but more is always better.
Next check your task manager to see where the problem might be. You can see what program uses the resources. To open the task manager, press alt+ctrl+del and choose the right option (I think you can also right click in the start menu and choose task manager in win7)
Other than that it may be your hard drive that s about to quit on you. If you consider changing the hard drive I suggest going with a solid state drive. You can find a 500 GB drive for about $150 and then you'll need a cloning kit that comes at $20.