Ok, so even after you re-installed windows, you're still using 90% disk space? Did you just re-install windows on top of what you had, or did you format? It sounds like you just reinstalled over what you had but likely this wouldn't do much good.
I'd honestly look to getting a second drive and copying some of your data to the second drive.
How large is your hard drive? Are we talking about 90% of 80GB or 1TB? And what about memory - how much is installed and what can it be upgraded to? Also speed of your CPU - six years old could be pretty slow by today's standards.
I may have worked fast in he past, but as software becomes more and more demanding (as it has) your CPU just can't keep up.