This has happened to me 3 times and to be honest i don’t actually think it is a problem with Avast. There is a school of thought out there that is has something to do with a recent Windows Update as it has happened to many Windows 7 users who do not use Avast. It could also be Malware from a recent website you have visited. I found this happened to me always after visiting a certain website, which i have now blocked with Avast. Have you been visiting any porn or gambling websites? I was always able to reboot in safe mode but to be honest that doesn’t really do much.

If you want to fix this all i can say is i hope you have made regular backups and saved an ‘image’ of your system because you are going to need it. Reboot and press F8. You should then have a list of options on of which being restore or something along those lines. You can restore from backup there. Don’t bother using last restore point as it probably will not work. You have to restore from a full system backup. Let me know how you get on.