Yes it only fixes the virusses just like the virusscanner does now. But only then without Windows 7 running. You can boot normally without issues. Unless it cures the problem incorrect. But they always say which files are cured and they have files in quarentine.
And you can always go back in time using System Restore
But it's just theorectically!!
- most of time no virus is found
- virus are cured the way it should
Never had any problem with offline virusscanners. Sometimes they can't fix it, or don't find the virus. But unbootable.... never