You have two things going against you.
1. Your CPU being a Celeron will always be a slow point on your computer.
2. Your drive slows down as it ages. This doesn't mean your drive is dying it just happens.
First use memtest to ensure your RAM's integrity. If that passes,
There are two things you can do.
1. Replace your drive by cloning it over to a new one or installing fresh from CD or USB.
2. Using a different computer to low level format your drive and install the OS from CD or USB.