Here is an update on what I ended up doing.
First and foremost, I got my Compaq Presario up and running! In fact, I'm using it right now to make this post
I didn't end up buying a new HD because I had a perfectly good HD in my daughter's broken Toshiba Satellite notebook. It had a broken LCD screen but everything else worked. Yesterday, I cannibalized parts off of it to bring two laptops back from the dead!
I got my older Averatec (XP!) computer running again (see this thread: Does this sound like a Recovery CD problem?
and I installed the Toshiba HD into this Compaq and loaded up the recovery disc. Both computers use SATA HDs with their own special connectors on the end of the HD that I had to swap.
Vista takes sooooooooo long to load. It successfully loaded and I've been using the laptop since yesterday evening.
I took three dead computers and made two good working notebooks out of them. I'm feeling super cocky about being a computer "mechanic!"