The problem you described is caused by a browser hijacking program.These programs are capable of changing your default webbrowser startpage, redirecting searches, displaying unwanted pop-ups, adding toolbars to your browser, etc. Often these trojans or spyware programs get downloaded when visiting a malicious website with Internet Explorer.
To remove the specific trojan you mentioned, you can run the following cleaner program:
Spybot is freeware and a free version of Ad-aware exists for personal use. Much like an anti-virus these programs allow you to scan your PC from time to time and specifically remove adware, spyware and certain trojans. Spybot also has the ability to immunize your PC against hundreds of known malicious programs. A scanner will stay resident in the system memory when you surf and will alert you if a browser hijacking is attempted.
This combo should block just about any trojan if you keep them up to date and run them regularly. Nevertheless, here are some alternatives:
This nifty program prevents spyware ActiveX controls from being
installed on your computer by setting your system so that these
controls are prevented from being installed or run. It also blocks
cookies from certain websites or companies that are known to use these cookies to track a user's surfing habits.
SpywareGuard runs behind the scenes to block spyware from being
downloaded or run on your computer. It also has the ability to prevent browser hijacking (whether it is your start page or your ability to search using a search engine or otherwise).
Ideally, these programs are combined with a good anti-virus. If you're looking for a free version, you might want to try the freeware version of AVG:
Besides viruses and worms, AVG also detects and removes trojans.
Finally, you might want to consider trying a different web browser.
Internet Explorer is very susceptible to these type of take-over
attempts, as it allows the execution of Active-X components of
webpages (unless if you explicitly disable them). This way, you may
download and execute malicious programs without knowing it. Browsers such as Mozilla Firefox are usually more resiliant to such attacks: