I've used it for a while now and my computer has run well ever since i first installed it. Basically it keeps your computer clean from files that might otherwise pile up after a while and cause errors,frozen screens and decrease the overall speed of a computer. Check it out,might be what you're looking for. I would then suggest that you do just that. Reinstall Windows again.
System file can easily become corrupt in that lenght of time.
I would suggest doing this atleast every year. Select properties and on the General tab click on Disk Cleanup. It will give you a couple of options and explain what each function does. It will have check marks next to items that should be cleaned up. Hit the okay switch and it will clean out your unused or overused files.