In many situations it is not acceptable. Here comes the importance of rescue system. There are programs like Avira AntiVir Rescue System that allows repairing the operating system that can not be booted anymore. Even if your computer is infected (or damaged) completely by a virus, you can repair it effectively as it operates complete independently.
Independently I mean, you need not install it on your PC instead just run from a bootable CD. Avira AntiVir Rescue System is a Linux-based application that allows accessing computers that cannot be booted anymore. With this free tool you can repair your damaged system, rescue data from your computer and scan the system for virus infections.
After download (60Mb), just double-click on the rescue system package to burn it to a CD/DVD. You can then use this CD/DVD to boot your computer.
The Avira AntiVir Rescue System is updated several times a day so that the most recent security updates are always available.(via –bloginformatico)