If you allready have IE7 then select your home page from Tools/internet options/general/then add your url for your chosen homepage highlight and select use current , you can also delete the tab directing you to IE7 updates page in the box by right clicking and deleting
Do you by any chance have Norton AV installed? This has a "protect homepage" option which will stop the changeover. The answer is actually in the Norton Antivirus options. Get there from the Options menu on the main Norton security window, then select Antivirus and the options window will appear. On the left you will see Home Page options. Within these uncheck the box which says "protect home page" and then OK etc. You can now change your homepage, and then repeat the process to recheck the "protect" box to hopefully stop it changing again.