First - ensure you do not have "Vundo".
It seems possible that your Windows installation has been deliberately
damaged by a trojan (Vundo, along any uninvited guests. (SDBot and ZLOB, all
protected by a rootkit.)) so as to prevent you updating your system or
removing the trojan.
Your first priority must be to cleanse your PC of all malware.
A very good antimalware app is SUPERAntiSpyware and Malwarebytes
Anti-Malware.
There is a free version (on demand scanning only)
<http://www.superantispyware.com/>
Reboot
-=-
Try running Malwarebytes Anti-Malware... download, update, and perform a
full system scan:
<http://www.malwarebytes.org/mbam.php>
Reboot
-=-
Good luck
Let us know what hªppens.
Ǝиçεl