I have recently installed a western digital pata 69 gig 5400 drive in my laptop. everything was installed from scratch Win XP pro. But the hdd light is always on. I ran all the WD diagnostic test you can download from thier site and everything passes.
My previous drive did not have this problem and the HDD light blinked when it was supposed to.
my computer is an Inspiron 7500
Do a speedtest with something like HDtach and make sure the drive is running as it should(speed wise, if it is real slow, then something may be wrong). as long as it is, it may just be a glitch with the drive it self showing activity when there is done.
Does it stay on when you are in the bios as well?
Is your drive indexing files for faster searching? Self Defrag maybe?
If it passes the tests chances are it is good. Signs of failure show up fairly early in SMART and WD Lifeguard reads SMART attributes.