Update your antivirus program with the latest files, and check for a virus.
If none, then d/l Ad-aware or Spybot, and remove all spyware.
If that does not help, and the problem happened within the last 5 days, then press and hold the Ctrl key during startup, select 'Command prompt only', press Enter, at the C:\> prompt type scanreg /restore, press Enter, select a Registry with a date that precedes the problem, and press Enter.
If that does not work or if the problem happened more than 5 days ago, then try reinstalling Windows over itself.
If still no good, then back up all critical files, reformat the HD, and reinstall Windows.