At first, LyX was popping up an error message (something to the effect that it couldn't find some sort of MiKTeX thing; I'm sorry I didn't write down the exact message). Anyway, I was able to fix that problem: I followed the instructions on How do I update my TeX distribution? (specifically, I used Method 2 under "MiKTeX GUI (Windows XP/Vista/7/8)") and installed all of the "installable packages" (there were lots). [In case it matters, I had to chose a new package directory first. Also, I didn't see the option to "Select Updatable Packages" and a few of the listed packages apparently weren't either "installable" or "uninstallable" (because neither the plus nor minus sign became activated when I selected those packages).]
Anyway, now when I try the pdf preview, I no longer receive an error message. Instead, it says "Successful preview of format: PDF (pdflatex)" in the bottom left-hand corner. But no pdf shows up! (I've tried a few different files, including LyX's "Introduction" file).
I have Adobe Reader 9 installed on my laptop (and no other pdf readers, to my knowledge). I am certain I was using Adobe Reader with LyX the last time I had LyX installed (just a few months ago), and it worked fine then. (Maybe I have allowed Adobe to install some updates since then, but I don't know).
One more note: When I re-installed LyX, it did say that the package directory I had been using was not functioning (corrupted or something like that), so I had to use a new one. I'm not sure if that could be causing an issue or not.