Sometimes, for some unknown reason, my HD light comes on indicating the HD is doing some thing. I can hear the HD reading and/or writing. I have nothing scheduled to run, no updates are down-loading (that I know of) so I always wonder, "What the heck is this thing doing?".
Can anyone tell how I can determine what is going on"

There are a number of possibilities, but a frequent one is the use of an indexer, either Windows or one of the other indexing programs, e.g., Google.
If you press Ctrl+Alt+Del the Task Manager will pop up. Look in the processes window to see if an indexing service is running.

Using the same procedure mentioned by quinland, while viewing the processes running, click on the image name column label to sort that field alphabetically, then look down and see how many instances of svchost.exe are running. Apparently, any program you install on your computer may use a separate instance of svchost.exe in its method of monitoring the internet for update services.
It has come to my attention that many software programs that you may install on your computer silently include a scheme to "contact the mothership" as a part of the installation. The incessant need to continuously check for updates and automatically install same will send your hard drive into activity simply because a variety of programs are monitoring for internet connections and on a predetermined cue from some hidden code, will activate the update search.
By watching the CPU column numbers within Windows Task Manager, you might be able to catch a glimpse of the offending process that is tying up the hard drive (higher numbers imply more resources in use). A word of advice though, if you have any hopes of catching the action in progress, I would suggested having the Windows Task Manager already up and running in the background BEFORE the hard drive activity begins because otherwise, there will be a major delay once the activity has begun.
Don't be surprised to discover that Symantec (Norton) or Sunbelt (CounterSpy) might be among the programs using all those resources if those are your internet security programs of choice.
While turning the auto-update features off may be possible for some programs, there are others that will not let you easily choose this option.

   Windows also has things that it likes to do when the computer is not busy, like creating a restore point or indexing (as mentioned above).