Appreciate all this info!

It's not a capacity problem - lots of free space.

I'm so nervous to try this HDD self test that Jo*Mar's 1st link suggests. Here's a portion: "It would seem there is a problem with the hard drive. You should go into the BIOS...tap F10 as you boot up and find the hdd self-test and run it and see if it comes back with errors." My excuse for putting it off is that the computer works fine. It's starting up that's the problem. Suppose I restart it and click to run the test and it's one of the times that the computer would've start up with no problem? (I'm asking myself this)

I'm such a wuss.