PC tower light

Hi Everyone.....this is not a terrible problem, just very annoying and I'm afraid it might be leading to something worse. First, the light on my pc tower ( the one that blinks when something is downloading) has been staying on all the time and only goes off when I touch the mouse to use the computer. Someone else told me this is normal for XP,that it's the disk defragmenter, but I've had this computer for 2 years and it's never done this up until a few weeks ago. Also, since that started happening, everytime I click on a desktop icon, it is very slow to come up. Once it does, the computer runs just as fast as always, pretty much. Normal for this computer is when I click on an icon, it's right there right away. Sometimes I have to click a couple times, even. ANy Ideas out there???????? Thanks