Long way
Short way
BUT, as noted by Ramesh, if you did not change the setting, it is most likely a sign of malware infestation and you need to IMMEDIATELY run a full scan for malware - be sure to update your scanners before scanning. I recommend you purge your system of clutter first using Windows (
XP or
Vista) Disk Cleanup,
ATF Cleaner or
CCleaner. If you use CCleaner, then during installation, uncheck the option to install the Yahoo toolbar and before first use, go to
Options >
Settings >
Advanced and ensure
Only delete files in Windows Temp folders older than 48 hours is
unchecked.
Note: Ensure you know your site credentials (user name and password) for sites you frequent before cleaning; you may have to login again at next visit.
Then download, install, update, and run
Malwarebytes's Anti-Malware (MBAM) to ensure your system is free of malware. Then do the same for all other computers on your network (everything on your side of the Internet gateway, typically a cable/DSL modem).