Jeff, there are a cumulative amount of reasons why your system has slowed down, filling up your hard drive to maximum is one ,
but more importantly your RAM is inadequate, forcing your programs to use virtual memory off of your hard drive and not
your RAM as it should, that is why when you first get your computer with hardly any programs installed and just your operating system
the computer seems pretty zippy. By the specs. that you've given I would estimate that the computer is @ 7 or 8 years old.
Being the age of the system that you have, adding more RAM or even finding the correct RAM that your mother board requires would be
somewhat difficult to find, since that RAM is most likely outdated. I'm with the other posters suggestion of replacing the whole computer with
something newer, its time to upgrade for now and the future and you'll appreciate the speed of the newer ones once you start working with it.
I consider myself a mid-level technician.