Go here http://www.sysinternals.com/Utilities/ProcessExplorer.html and get Process Explorer. Download it and install it. You might be surprised at what is running.
My hard drive has gotten progressively slower through the years, and despite multiple potential solutions, nothing has worked. Whenever I try to open a new window, change web page, or open a new application, the HD light is on solid for at least 15 seconds at times for something as simple as opening up "my computer" with no other apps open at the time. My HD was full, so I figured that was part of the problem, so I bought a new bigger HD, but no. Here are the things I have done to try to remedy the problem:
Buy larger hard drive (currently 70% free)
Search for bad sectors
Run Norton Anti-Virus
Clean Registry (CCleaner)
Clean up unused programs
I don't know what else to do!?! Any help would be greatly appreciated.