Generally this type behavior is by a stealth installation of a Browser Hijacker / definition http://www.bleepingcomputer.com/tutorials/tutorial41.html
Traditionally these are from a toolbar installation either by self or "drive by" stealth installation (occurred without your knowledge visiting a bad malicious website). They employ what is called an BHO Browser Help Object active x key in the Windows registry along with any other files.
They are generally always easily removed and the danger is they may be spying on all your browsing habits and transmitting that out for nefrious uses by cyber crime or others. This is why everyone jumps on getting rid of threats from their pc anymore. Aside from that, they introduce instability into the system as they are malicious malware and not legitimate operating or software systems usages.
Recommended to install, update, and run full scans by both a quality antivirus and antispyware program. See Microsoft Essentials to get the former Windows One Care Antivirus free now which is West Cloast Certified and has won the coveted VB100 Award ! FULL real time protection like pay products.
About Microsoft Security Essentials
http://www.microsoft.com/security_essentials/
Hey while you are here getting rid of threats - do the quick spring cleaning of your system. when you have a quality antivirus that has top scores at VirusTotal for detections and preventions (Norton, Sophos, Ikarus, Trend Micro, etc) - you are good to go. It is always recommended to try a couple antispyware programs. Some get what others miss and only because of discovery time of their catagory threats whereas antivirus researchers discover viruses and worms within the hour sometimes. TRY ALL THESE !!
Microsoft AntiSpyware is now Windows Defender 4-5*Stars
[working-freeware from Microsoft]
http://www.microsoft.com/athome/security/spyware/software/default.mspx
SUPERAntiSpyware [working-freeware, and premium version] 4*
http://www.superantispyware.com/
Google Pack with PCTools Spyware Doctor 4*
http://pack.google.com/intl/en/pack_installer.html?hl=en&gl=us
a-squared trojan remover (Free Working Version for life and Proactive Premium Version)
http://www.emsisoft.com/en/software/free/
a-squared (a-squared) is a complementary product to antivirus software and desktop firewalls on MS Windows computers. Antivirus software specializes in detecting classic viruses. Many available products have weaknesses in detecting other malicious software (Malware) like Trojans, Dialers, Worms and Spyware (Adware). a-squared fills the gap that malware writers exploit.