Oh yes, technology can be so "fun" at times, it just makes my mouth hurt to smile to much.........NOT!!
Okay, here's the condensed version of the problem and what I found to be the solution in my case. I'm posting this here in case anyone else runs into a similar issue, but I can't promise that the same solution will work for you.
I had to re-install Windows a few weeks ago (long story behind THAT bit of "fun"), and for the most part things were humming along just fine as I gradually re-installed stuff. However, somewhere in the past week -- maybe a little more -- I noticed that the system performance would occasionally take a big hit...and stay hit. Looking at the Windows Task Manager, I saw that the CPU usage meter was hovering up around 75% or so, even though the main view showed that the System Idle process was the one supposedly taking up 99% of the CPU. In short, there was clearly something very un-idle cranking away, but Task Manager wouldn't show it to me.
After a number of Google searches, I came across a site that recommended using a little-known program called Windows Process Explorer to take a deeper look at system processes. The moment I ran it, I immediately saw the problem. Apparently that 75% of the CPU time was being taken up by hardware interrupts. More Googling led me to a number of possible solutions to that particular problem, but most of them didn't help. However, I finally stumbled across a comment in an NVIDIA forum thread about NVIDIA IDE drivers sometimes causing problems (apparently they're optional drivers, and sometimes have a hard time operating properly when the computer comes out of sleep mode, which I use a LOT as an alternative to fully shutting down). I dug into the Device Manager and found an entry under IDE ATA/ATAPI controllers named "NVIDIA MCP51 Serial ATA Controller". I right-clicked on it, chose "Disable," and immediately the CPU meter in Windows Task Manager dropped to near zero. Woohoo!