I have had the same problem with my HD running for long periods of time. I too have read over and over on several threads about indexing and windows search.
One big, probably the biggest, contributor to my HD running was the Desktop Windows Manager (dwm.exe). From what I read the DWM continuously creates thumbnails of the pop ups on the task bar. Once I disabled this option the HD activity stopped.
I went further to review my services and my msconfig to review which services were running automatically and started at start up.
This path worked for me and for now the HD is quiet as a mouse.
Oh, I will point out that with the indexing off I have seen some delays in the search, which is to be expected. For the most part I know where things are on my PC.
I hope this helps.