I just installed 4.7 Home, all was working just fine till then, but Kasperty was expiring and avast was recommended. I am using Windows XP and completed the installation smoothly. It was scanning, I went for coffee and upon my return, ny computer re-booted, and re-booted continually. I tried a manual shut down - no change. I cannot stay on long enough to delete 4.7, or even a system restore. I am not computer literate, so, if anyone can help, please keep it simple - like explaining to your mom.
Two anti-viruses will conflict with each other. I think leftovers from Kaspersky are conflicting with avast.
First boot into safe mode & uninstall avast. Boot into normal Windows. Next follow the instructions on using the Kaspersky removal utility. http://support.kaspersky.com/faq/?qid=208279463 Reboot.
To be sure to get a clean install of avast, run the avast uninstall utility. Reboot. Now reinstall a fresh copy of avast.
Welcome to the forum. 
I am not computer literate, so, if anyone can help, please keep it simple - like explaining to your mom.
Here is how to boot into safe mode: http://www.pchell.com/support/safemode.shtml
He did say what OS he is using… Windows XP. Home or Pro version makes no difference in booting into safe mode. When you boot and the main bios screen is done showing you it’s setup, keep pushing the F8 button above your normal keyboard keys. That will give you a menu in Black & White that you can choose Safe Mode, among others.
This happened to me, I had kaspersky and I installed bitdefender antivirus and my computer kept on rebooting.I resolved this problem by reinstallting my entire Os.