I actually just now upgraded my hard drive just over 2 weeks ago. I'm also using a Dell Inspiron 1150. Upgrading the hard drive is the easiest thing in the world to do with a laptop, thankfully. All you do is on the left side, near where the Esc and F1 F2 F3 keys are, on the underside there are 2 screws. Theres also like a port where you might put in a wireless card. That area is the hard drive. You unscrew the 2 screws and you pull it out, then remove the 4/6 screws that secure the hard drive in the caddy.
Let us know how it goes.