I then looked at my applications and processes with the task manager--nothing was out of the ordinary or taking up loads of memory. However, I was informed by the manager that my CPU usage was at nearly 100%, even when I wasn't connected to the internet.
I scanned with both Windows Defender and AVG for Rootkits and Viruses/Malware, disconnected from the internet, deleted Temp files, defragged. AVG didn't find anything, but Windows Defender found a bunch of unknown errors with no description of any kind, but it passed over them. Defragging and deleting Temp files only cleared about 2 gig; in all, this mysterious problem ate almost 20 gig before it stopped. When it did, my CPU usage went down to normal.
After doing all of this, I'm afraid my computer knowledge has run out. There have been no error messages of any kind, and I have only downloaded one thing: a trial version of Photoshop CS5 (which is about 1.5 gig, but still isn't nearly 20). It occurs to me that I could've downloaded an infected copy, but I highly doubt it as I downloaded from the Adobe website.
I am usually not connected to the internet, but I have been for an oddly-long amount of time (2.5 weeks). I am losing the connection in a week. I don't know if this information is relevant, but it doesn't hurt to say.
Though the problem has stopped for now, I have no idea if it will start again, and the space is still being taken up. I would really appreciate that space back (as I am a hobby photographer and writer and this is my only computer) and the knowledge of what happened so I can prevent it and/or avoid it in the future.