Normaly, a virus try not to prevent you from using the computer, as it prevent the virus from doing it's work. Conversely, some ransom-wares will prevent you from using the computer, with the author hoping that you'll pay him to regain access to your files. DON'T pay! The author is NOT trustworthy, and you will loose your money, probably for nothing.
You may have a faulty or almost dead main drive.
I've seen similar problems caused by faulty RAM.
In both cases, try to boot from a linux live CD.
If the boot process works, copy all accessible files to some external drive.
It you can't boot, those CD normaly have some memory test. Run it. If the memory is faulty, you'll see some errors messages or report. Replace any identified faulty memory stick, even if it only show a single faulty address. If you have more than a single memory stick, your computer will work normaly, probably a little slower, with the faulty stick removed.
If you need to replace your RAM, take the occasion to upgrade it as much as possible.
If the memory test CAN'T work, it probably mean that you have some serious hardware failure, maybe in the BIOS itself. You need to replace the computer. The drive is probably still OK and the files intact. Install the drive as a secondary drive in another computer.
Maybe someone was over-zealous about cleaning the drive to recover space and suppressed some vital files. In this case, you need to repair or re-install the operating system. Your symptoms suggest that any restore points are effectively lost. COPY all user files to an external drive. Also, copy any file that you are not sure about.
You don't need to copy the following folders:
PerfLogs, Windows, Programms (or programs or program files) and any folder that looks like it related to your anti-virus.
In the users profiles, you don't need the "Default" folder. (it's only a template used when creating new accounts)
If you have Windows Vista, 7 or 8, you can use the "Update" option from the install DVD. It's designed to preserve all user data and installed programms. It's still prudent to backup your data.
If you have Windows XP, you need to reinstall from scratch. You absolutely need to backup your data.
Electro, December 2012
