Mostly PC are infected with browser hijackers which can neither be detected nor removed easily, I am going to introduce the software which can do so
HijackThis is tiny program examines vulnerable or suspect parts of your system, such as browser helper objects and certain types of Registry keys. Pressing the Scan button generates a log of dozens of items, most of which are just customizations. Don’t check off an item and hit the Fix Checked button unless you’re sure it’s malware. Clicking Info on Selected Item tells you why the entry was flagged as suspicious, but not whether it’s actually malware. To find that out, search the Web for that item’s name or go straight to a forum, such as Spyware Info or Computer Cops. Saving the log creates a text document you can post to these forums. The latest version adds powerful tools to the Config window. The process manager and hosts file editor help you excise virulent infections. The unique ADS Spy tool scans for alternate data streams, which some browser hijackers use to hide from spyware removers. The program still installs into whatever directory in which you unzip the file, which can make it hard to locate. HijackThis is a serious tool for any user who needs to root out a serious infestation, but wield it with caution.
You can download HijackThis from here