It really depends on what you do with your computer as to what's going on. Some programs do a lot of background chores leading to a little bogging down of the system.
If you have toolbars that go out and find things like the weather, that ties up resources that might be needed elsewhere.
If you use online chat services like yahoo or MSN, and leave them turned on and logged in then that too will use up resources.
If you have programs that start up when the computer starts up add that to the list. Those are sitting in a standby mode burning up resources.
If you use a resource hungry anti virus program like Norton, you need to ditch that POS and find something else that is not so resource intensive like maybe ESET's NOD32.
And finally some out of mainstream cleaner and defragger programs have hidden assets that phone home or are just poorly written and don't do what they are intended to do correctly.
Seti's toolbox is what I use and a lot of the members here also. I've used every one of those products and don't take recommending them lightly. PC Cleanup Toolbox
You need to take a hard look at how you use your computer.