Shoutbox

A really useful program for this is Hijack This

When you run it, a list of of files and keys will appear. There will probably be one like this:

HKCU\Software\Micorosft\Internet Explorer\Main,Start Page = <the site that your internet now starts at when hijacked>

Check this key and click fix checked.

Do not delete anything unless you are completely sure it is something bad

I also recommend trying CWShredder which runs a scan and is really good.