PLEASE HELP WITH MAC OS X PROBLEM! Harddrive is locked?

I am so frustrated and confused. Here's what's going on:

I have an older grey Powermac G3 with Mac OS 10.1 on it. I am trying to upgrade to 10.1.5 or later, but it tells me I need a new version of Installer. So I downloaded the Apple Installer update, and when I try to install it, IT WON'T WORK. When I go to choose a destination, my harddrive is greyed out, and I can't select it. Other applications tell me that it is locked. WHY IS MY HARDDRIVE LOCKED???!! I am going crazy trying to figure this out. I've tried everything. Does anyone out there know why this might be happening and/or what I can do to unlock my harddrive?

It's weird. Even though I am the admin and there aren't even any other users on the computer, some stuff like pulldown menus and options and stuff are greyed out. Shouldn't I be able to access everything? My neighbor suggested that there might be a problem in the harddrive and it's locking down for some reason...but I don't know!

Please, does anyone know what's going on?