Put your HJT logs in space provided on site and it is analyzed for you.
http://www.hijackthis.de/index.php?langselect=english

or go download HijackThis:
A general 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. As a result, false positives are imminent and unless you are sure what you're doing, you should always consult with knowledgable folks (e.g. the forums) before deleting anything.

Mirror links:
http://www.merijn.org/files/hijackthis.zip
http://downloads.subratam.org/hijackthis.zip
http://www.unitethecows.com/software/HijackThis.exe

Tutorial:
http://www.merijn.org/htlogtutorial.html

*Make sure you read and fully understand the results of the analysis before removing anything suggested.

This is by far the most poerful antispyware tool. But... is user-unfriendly, only experienced people should use it.

If you don't know what to do: make a log and post it on a specialized forum. It's the best way...