Computer running horribly laggy, audio distorted, CPU stuck at 50%

Hello,

I upgraded to Windows 8 about a month ago and it was running beautifully and fast until yesterday. After a road trip home, I set up my computer and woke it from hibernation and suddenly the entire computer ran ten times slower (this applied to everything), and audio, including music and system sounds, were all fuzzy and in slow-motion. I haven't bothered trying videos or games.

Please note that I am not a computer newbie; here is what I was able to figure out. My fan is running at low speed, and the computer is not hot (CPU is around 60C), but to be safe I disassembled everything and cleaned it. This did nothing to help. The fan is not broken because when I boot it goes to full speed for a few seconds. So it's not a heat problem.
Next I tried my hard drives. I did a full chkdsk for both of them and they're fine. I also defragmented both of them, still didn't work. I found something on the internet about a similar issue occurring to drives that reverted from DMA to PIO mode, but they're both SATA drives and have no DMA option. So it doesn't seem to be the drives.
Fairly certain it's not a virus, because my antivirus hasn't detected anything, and I have a lot of experience with viruses; this doesn't look to be one.
Sometimes when the computer boots, it runs fine for a few minutes and then inevitably goes back into lag mode. But I haven't found any rogue processes, and I don't recall installing or changing anything in particular recently, and especially as this started after a hibernation, I have no idea what's going on.
Eventually I used System Restore to go back about 5 days, but lo and behold it still booted up laggy. I want to avoid reinstalling the OS if I can, especially as I'm not sure if it would fix it.

Here are some screenshots of my performance monitor and process explorer windows when all unnecessary processes are shut down (note my computer is in Japanese, but it should be understandable): http://imgur.com/a/DomrZ
Notice that the CPU is stuck at about 50%, and the processes that use the CPU are pretty much random. If I kill something that is using most of the CPU, then something else starts using the rest of it.

My computer specs are:
HP Pavilion dv8 (computer is 3 years old)
Windows 8 Pro
Intel Core i5 M430 2.27GHz dual-core with hyperthreading
8GB DDR3 RAM
NVIDIA GeForce GT 230M 1GB GPU

My computer is almost unusable now, so I hope someone can help me.

What happens if you boot to Safe Mode or Safe Mode with Networking (try both)? If it runs better, see here:

How to perform a clean boot to troubleshoot a problem in Windows 8, Windows 7, or Windows Vista

I tried two things: First I tried doing a clean boot (all startup programs and services unchecked) and the problem persisted. Then I tried booting Ubuntu from a USB stick, and that seemed to run just fine. I don't want to use Ubuntu though so I still have the same problem.

I found the problem! Despite my computer being plugged in, my DC port wasn't working properly and didn't detect that the computer was plugged in instead of on battery. So I changed the power setting to High Performance, and the computer stopped switching to power conservation mode (which caused the horrible lagginess). Everything is a learning experience.

Good find though, and thanks for letting us know.