I am going to assume that the LED you are referring to is the HDD activity LED. You can tell by loading some big video then seeing if the light becomes drastically more accurate while the video is loading.
First off, HDD activity is not necessarily a bad thing, but being interested in maintaining your system is a good practice as if you become keen at it you can keep your system running smoothly and also see tell-tale signs of when hardware may fail.
1) If your OS is Windows XP and you haven't re-installed for years, do this first. Windows XP is terrible for keeping itself properly maintained, and after dealing with it for over 10 years, I have come to discover there are things so ingraned in it that cause "bit decay" the simplest method for maintenance is periodic re-installation.
2) If your OS is Windows Vista or higher (7) then your system should properly maintain itself, Windows 7 does it better than Vista though. Starting with Vista HDD Defrags always occur in the background every couple days or so. What would be causing excessive reads at this point would be applications or malware running processes you don't expect in the background, or possibly a failing HDD. If your HDD is failing it increases in activity as it has to re-read parts of data more and more.
3) Your system could be doing work that is actually reasonable. An example is if your Windows system is checking for updates, it will need to reference what is on the system before it can determine what updates you need. As a precaution best just update Windows completely.