[ Other - Computers ] Open Question : My computer slow, it used to be lightning fast...help please :)?

I have Gateway Dx-4200-09 Vista desktop. I have 455 GB of free space in my hard drive, more than 75%. Recently it slowed down lots, to the point Internet Explorer thanks minutes to load. I ran Norton, AVG, System Mechanics. No spy wares found, no system errors. I don't know what to do anymore. I un-downloaded anything un-needed. Please help ^-^