My CPU is running with 100% power

Hello! I have a seroious problem I'm trying to get rid of for a month. Task manager is showing that my CPU is running at 80-100% when I'm not even doing anything. My whole PC is lagging so much I can't do anything, and when I try, it takes some time. And what's funny: Anything in Task Manager isn't consuming that much CPU. I don't know what to do now. I tried to scan my system with various sofware, AdwCleaner, ComboFix, Malwarebytes Anti Malware, AVG Internet Securinty 2014 etc.
I don't know if this is important, but before this problem, I had a problem with BAD_POOL_HEADER BSOD, but with some research I knew that AVG was the problem, so I re-installed it, and since the first boot after the BSOD my CPU is running like crazy. I even tried uninstalling whole AVG, but nothing happened. I know, probably the first answer would be "Try to reinstall Windows", but I can't at the moment. I had original Windows 7 Home Premium 64 bit, but I can't find it. I was searching everywhere, but nothing.

Please, is there any way to find out why my PC is running like that?

Sorry for my english, this isn't my native language. Hope you understand.

Can I do this in safe mode, or do I need to boot my PC in normal mode first?

Ok, I ran SFC /SCANNOW, but nothing. It says that nothing was found. I'll try some other things from tutorial.
EDIT:
Oh, I forgot to say that - I already tried clean boot in msconfig and system restore, but nothing.
EDIT2:
I noticed one more thing. There is one Windows Update that somehow can't be installed. Everything is fine, but for some time this particular update won't install.

I don't know what happened, but my problem solved itself. I was fighting with it for a month, but a random reboot just solved it. I'm confused. Anyway, thank you very much for your help :) 12% CPU usage at the moment!

Yeah, I wanted to do some stuff from the tutorial, but first I just decided to restart my PC... And it's working just fine.