I was recently presented with a problem by my in-laws. Their browser's homepage kept on resetting to MSN every time they restarted. Although a reasonable homepage to have, they prefer to just have the google.co.uk front page.
The specific page that the browser would reset to was - http://uk.msn.com/?ocid=iehp. I thought the same as you, just go into options -> properties and reset the homepage. Nope. Every time the browser restarted, back to good 'ole MSN.
So what is the solution? Well, after much googling for various things, I found an obscure reference to my dilemma on a forum (can't remember which, sorry). It turns out that some spyware and antivirus programs (in this case Norton) have 'homepage protection'. This setting locks anything from hijacking your browser homepage for their own nefarious purposes. It also means that once a homepage is set, it will continue to default back to that upon a browser reload.
So there you have it. Turn off your homepage protection, and return it to the correct homepage. I don't know what happens if you switch the homepage protection back on and I don't intend to find out.