Are you logged on as an Administrator? If not, make sure you are for the rest of my ideas.
First verify you are virus-free, some viruses try to block installs. Download TDSSKiller and RKill from our
Freeware DB, if neccessary, do this on another computer and put them on a USB flash drive. Then reboot your system into safe mode without networking (Reboot and, when it says "Starting Windows," Press F8; If you get a message asking continue in safe mode or perform a system restore, choose continue in safe mode). Now run TDSSKiller and after that run RKill and after that run a full virus scan (Hopefully you have an anti-virus installed because you will need it so if not let us know and we'll suggest an alternative method). The first two will take maybe 5 minutes each but a full virus scan can take anywhere from 30 minutes to several hours depending on your hard drive size and number of files.
TDSSKiller is designed specifically to look for rootkits. RKill is designed to stop the stealthing programs that hide/protect/regenerate some nasty viruses. Running these first will help ensure your A/V has the absolute best chance of cleaning up infected files.
Assuming you are virus-free, try this
Fix-it from Microsoft. NOTE: any Fix-it programs that charge money for this problem are NOT from Microsoft and I suggest you avoid them.
If that doesn't work then try Step 2 in the answers
HERE.
