I use the free version of Avast!. So up until today, my boot time scan has been working flawlessly. Today, it kept stopping at 9 or 10% while it was scanning through the fonts on the computer. So I decided there must be a glitch in the system. I uninstalled and then reinstalled Avast! after the scan failed on me a few times. That didn’t work. So I called tech support and they told me I’ve got some sort of major virus on my laptop that’s keeping the boot time scan from happening and what they could do to remedy it, I had to pay for. But I didn’t want to pay, so I decided to give my laptop a system recovery and wiped it clean (no worries, there’s nothing that important on the laptop). So I re-installed Avast! and ran another boot time scan. That failed too. What kind of virus is on my computer and what can I do about it? It must be really strong if it could survive a system recovery. I feel like my laptop is waiting to die on me 
follow the guide. http://forum.avast.com/index.php?topic=53253.0
Yeah…nothing seemed to work. My boot scan still isn’t running the whole way through…
The main tools on that thread are for analysis to catch what the automated systems miss…