Sounds like the Google Redirect spyware. Try rebooting into 'Safe Mode' by tapping F8 repeatedly while booting up and then try using System Restore to go back to before all this happened.
I'm guessing as I have no idea what your operating system and service pack is but try Start > All Programs > Accessories > System Tools > System Restore.
If it works you will need to turn off Ssystem Restore afterwards to get rid of the infected restore point then turn it back on again and you will also need to update Windows and McAfee.
Unfortunately antivirus software isn't much use against these things that rely on you clicking a bad link.
Tell me of that works.