I Need Help!!

I'm having some serious problems with my computer and i need help. I'm not computer genius, but if you walk me through it i can figure it out. A couple of weeks ago i started having pc repair pop ups non stop. Then my AVG crashed i tried uninstalling it and it told me windows can't access the file and that i may not have the appropriate permission to access the item. A few days later my yahoo messenger crashed too giving me the same error message. I also now get this message when i start my computer... the box shows C:\WINDOWS\system32\pmkjj.exe with the same error message as the other 2. I tried installing a different anti virus and still get the same error message when i go to install it. I don't know what to do. Any suggestions?

 

And now it has crashed my iobit smart defrag.