What you are hearing is disk thrashing.
It is very noticeable in Vista operating systems.
One cause (and usually the culprit) is superfetch, especially during start-up. Many have disabled and noticed no performance decrease.
Before following the below steps, you might want to read up on superfetch before disabling it. It is reversable though.
Click on the Start orb and type services into the Start menu search box. Next open services and scroll down to Superfetch and double click to open. Next to "Startup type" in the drop down box select "Disabled". Apply the change and close out of Services. You can also disable "Windows search" in services to reduce thrashing even further.
Once you have close out of all open windows reboot your pc and see if the disk thrashing still occurs.