HijackThis

Product: HijackThis 2.0
Company: Merjon
Website: http://www.spywareinfo.com/~merijn/
Type: Free
System: Windows 98/Me/2000/XP

HijackThis is a homepage hijackers detector and remover. 

Initially based on the article Hijacked!, but expanded with almost a dozen other checks against hijacker tricks, it is continually updated to detect and remove new hijacks. It does not target specific programs/URLs, just the methods used by hijackers to force you onto their sites.