I have repaired a many large number of Windows PCs in my time. With the advent of newer more complex viruses & malware, which dig really deep into the operating system (such as the Antivirus Live, and other rogue/fake antivirus malware) & when large amounts of damage are left behind after viruses &/or apps are removed/ripped from the OS, I found running ComboFix is an absolute savior.
ComboFix is able to remove really bad virus & malware infections the get super deep into the OS, and can fix some really major Windows damage that results from other solutions removing/damaging OS files/apps, where even normal anti-virus, anti-spyware & other specialty Windows repair tools cannot correct the underlying problem.
I do not recommend using ComboFix as your first choice, unless you know what you are doing. But recommend saving this as your absolute last resort, before giving up completely on a given machine.
Simply download ComboFix from the "bleepingcomputer.com" web site & follow their noted directions. Sometimes a machine is too far gone to be salvaged. But in many cases, it is able to get a machine working again & make it secure/stable enough enough for data recovery & backup purposes.
However I give a large word of warning... Even though a machine was salvaged after being repaired with ComboFix, I find that over time that Windows installation will likely break down due to other problems. As such, I recommend using ComboFix to simply revive Windows & bring back to working condition. Then backup your data/settings, and then rebuild your OS from scratch (i.e. do a complete hard drive format, re-install Windows and then re-apply your apps from scratch). Even though this is more work then most people are prepared to do, this will give you the best overall result. As it will prevent any lingering issues that ComboFix could not completely repair from messing you up at a much more importune time. At least in this way, ComboFix should give you enough breathing room & time to schedule your system rebuild properly in the very near future, when its more convenient for you.
* DISCLAIMER: We at BuildDesignCreate.com cannot be held responsible for any damage &/or misuse of this application. Use at your own risk. As running this program without supervision can cause your computer to not operate correctly. Therefore only run ComboFix at the request of an experienced PC repair tech or upon an issue of last report (i.e. you have given up all hope & are going to format your hard drive anyway, so what the hell....)