It's possible some kind of infection has taken over so follow the instructions in this "Required Reading" first.
If that doesn't help try installing the FREE version of THIS tool, update it and run a full scan and let it remove anything it finds and reboot immediately if it asks you to.
Should your browser not let you go to that page try doing all that in "Safe Mode with Networking" reached by tapping F8 repeatedly while booting up and choosing that option from the ensuing menu (it works fine in that mode).
Should this infection not let anything be downloaded or installed try first "Save As" the installer to your desktop and renaming it in the process to soemthing random such as abcde.exe or 12345.exe for instance.
Lastly it's very important that you keep your operating system, whatever it is, totally up to date, along with all it's associated asoftware and applications.