Slow computer or virus?

[sunshine&flowerpots]

Hi,

My computer has been running slow for a while, I suspect something is in the background but wasn't sure until the last few days. I've noticed that when I left the computer on overnight (I shut close all apps, then turn of the screen, but leave the computer on) the last few mornings when I have gone to my computer, the machine has been running really fast - sounds like it's going to take off & the CPU is running at 100%. I've had to shut it down completely to stop it.

I've ran the FRST but I'm having trouble running the aswMBR - I get a system error message - Device has ran into a problem and needs to restart. I've tried re-running the app & have the same message each time. I update the Avast, then let it run. The computer closes, restarts as normal. Then nothing.