When doing anything that was memory intensive, such as opening a large file on the computer, programs would slow down by quite a bit, and the mouse cursor would become very stuttery and skips a lot of movement frames. None of this had been happening before a few days ago, but I don't really know what could have caused it. A post on the Malware Removal board has pretty much removed malware as a possibility, but I can't really think of much else myself.
As for some other details, I noticed something very odd when checking Windows Task Manager. When doing much more than idling or just typing, something weird happens where the currently used CPU percent becomes higher than it should be. For example, if System Idle Processes was at 80, the current CPU usage would say something like 44%, when it should be around 20%, and the currently used CPU for other processes would total around the 20% instead of the 44% it said was in use, if that makes sense.
Anyway, when the "unaccounted-for" amount of CPU percent gets high, that's when the slowdown happens. It could be something like slowdown happening with 50% CPU currently used with only 10% of that accounted for, but little or no slowdown if it's at 95% used with nearly all of it accounted for.
It's getting really annoying to deal with, since I have to avoid doing things that are usually pretty normal on this computer, and I'd really appreciate any help someone can give me.