That is not avast what you called but a third party (as has been explained on this webboard many times).
GrimeFighter changes the boot.ini file so it will load GrimeFighter and not Windows.
Undo the change and Windows should boot again.
- Boot from a live cd/dvd/usb
- Open the boot.ini file
- Change the content
- Save the changes
- Reboot the system (don't forget to remove the cd/dvd/usb)